Inventory planning connects expected demand with the stock, cash, space, and supplier capacity needed to serve customers. This glossary defines 80 terms, from basic stock classifications to financial measures, replenishment concepts, planning documents, and the ERP and WMS systems that support inventory decisions.
The vocabulary persists because inventory crosses several functions. Merchandising, purchasing, finance, warehouse operations, sales, and supply planning may use different terms for related decisions, so a shared definition helps teams interpret the same SKU, forecast, order, or stock position consistently.
Why Does Inventory Planning Vocabulary Matter?
Inventory planning terminology gives teams a common model for deciding what stock is needed, where it should sit, and when it should move. Clear definitions reduce ambiguity between demand forecasts, purchasing plans, warehouse records, and financial reports.
Inventory planning starts with the relationship between demand, supply, time, and cash. A planner who distinguishes on-hand inventory from available-to-promise inventory, or cycle stock from safety stock, makes a different decision from one who treats every unit as interchangeable.
Terms also describe different levels of the operation. A SKU identifies a sellable item, an ERP records cross-functional transactions, a WMS records warehouse activity, and WSSI brings expected sales and stock positions into a planning view.
Which Inventory Planning Terms Describe Demand and Supply?
Demand and supply terms describe what customers may need and what the business can provide. These definitions help planners separate a forecast from an order, a commitment from a possibility, and a supplier promise from an internal target.
- Demand: The quantity customers are expected to want during a defined period.
- Demand forecast: An estimate of future demand based on available commercial and operational inputs.
- Independent demand: Demand driven by external customer purchases rather than by another production requirement.
- Dependent demand: Demand calculated from the need for another item, assembly, or production plan.
- Demand signal: An observed input, such as an order, sale, promotion, or market event, that informs planning.
- Seasonality: A recurring demand pattern associated with a calendar period or commercial cycle.
- Demand variability: The degree to which actual demand changes around an expected level.
- Supply plan: A time-phased view of how demand may be covered through inventory, production, transfers, or purchasing.
- Supply variability: Changes in the timing, quantity, or reliability of incoming supply.
- Lead time: The elapsed time between placing a supply request and receiving usable stock.
- Supplier lead time: The time a supplier takes to produce, prepare, and deliver an order.
- Transit time: The time goods spend moving between locations.
- Available to promise (ATP): The quantity that can be committed to a customer after considering relevant stock and planned supply.
- Capable to promise (CTP): A commitment based on available production or supply capacity rather than finished stock alone.
- Confirmed supply: Incoming stock supported by an accepted order, schedule, or supplier commitment.
- Allocation: The assignment of available inventory to a customer, channel, order, or location, including prioritization when stock is constrained.
- Backorder: A customer order that remains open because the requested stock is not currently available.
- Stockout: A condition in which the required item is unavailable for a requested use or sale.
- Fill rate: The proportion of demand fulfilled immediately from available stock, measured according to a defined unit, order, or line basis.
- Service level: The target probability or performance standard for meeting demand without an unacceptable shortage.
What are the Main Stock Types and Inventory Classifications?
Stock classifications explain why inventory exists and how it should be evaluated. A single warehouse may hold saleable goods, components, partially completed items, returned units, and stock that no longer supports current demand.
- On-hand inventory: Physical stock recorded as present at a location.
- Available inventory: Stock that is usable for a defined purpose after exclusions such as reservations, quality holds, or damage.
- Reserved inventory: Stock assigned to a customer, order, channel, production job, or other commitment.
- Cycle stock: Inventory used to cover expected demand between replenishment events.
- Safety stock: Inventory held to absorb uncertainty in demand, supply, or timing.
- Buffer stock: A broad term for stock held to protect an operation from a known risk or interruption.
- Anticipation inventory: Stock built ahead of an expected demand increase, event, promotion, or supply constraint.
- Pipeline inventory: Stock that is on order or in transit and has not yet arrived at its intended location, tracked under separate on-order and in-transit statuses.
- Consignment inventory: Stock held by one party while ownership remains with another until an agreed transaction occurs.
- Raw materials: Inputs purchased for use in manufacturing or assembly.
- Work in progress (WIP): Items that have entered production but are not yet finished.
- Finished goods: Completed items ready for sale, shipment, or use.
- Maintenance, repair, and operations (MRO) inventory: Supplies used to operate or maintain facilities and equipment rather than sold as finished goods.
- Slow-moving inventory: Stock with a low rate of consumption or sale relative to its holding period.
- Deadstock: Inventory with little or no expected demand under the current plan.
- Excess inventory: Stock above the quantity justified by the current demand and supply plan.
- Obsolete inventory: Stock that no longer meets a product, market, technical, or commercial requirement.
- Stock keeping unit (SKU): A unique identifier for a sellable item, used to track stock, sales, and orders at item level.
- Quarantined inventory: Stock isolated pending inspection, testing, documentation, or disposition.
- Perishable inventory: Stock whose value or usability declines with time, temperature, age, or freshness.
How do Planners Manage Replenishment and Purchasing?
Replenishment terms describe the rules and documents used to restore stock. They connect inventory policy with purchase orders, production schedules, transfers, minimum quantities, and supplier timing.
- Replenishment: The process of restoring inventory after demand, consumption, or planned stock positioning.
- Reorder point: The inventory position at which a replenishment action is triggered.
- Reorder quantity: The quantity requested when replenishment is triggered.
- Min-max planning: A policy that triggers replenishment when stock falls to a minimum level and orders up toward a defined maximum level.
- Order-up-to level: The target inventory position used to calculate a replenishment quantity.
- Economic order quantity (EOQ): A calculation that balances relevant ordering and holding costs to inform order size.
- Minimum order quantity (MOQ): The smallest quantity a supplier accepts for an order or item.
- Order multiple: A quantity increment in which an item must be ordered or supplied.
- Purchase requisition: An internal request to obtain goods or services.
- Purchase order (PO): A formal buyer document specifying an order for goods or services.
- Goods receipt: The recorded arrival and acceptance of ordered goods.
- Allocation plan: A plan for distributing incoming or available stock across locations, channels, or customers.
- Stock transfer: The movement of inventory between company-controlled locations.
- Drop shipment: An arrangement in which a supplier ships an item directly to the customer on behalf of the seller.
- Vendor-managed inventory (VMI): An arrangement in which a supplier participates in monitoring or replenishing inventory at a customer location.
- Material requirements planning (MRP): A planning method that calculates material needs from demand, bills of material, inventory, and supply timing.
- Distribution requirements planning (DRP): A planning method for positioning inventory across a distribution network.
- Enterprise resource planning (ERP): A system that records cross-functional transactions, including purchase orders, sales orders, invoices, suppliers, and inventory valuation.
- Open order: An order that remains active because its expected quantity has not been fully received or closed.
- Expediting: An intervention intended to accelerate delayed or time-sensitive supply.
Which Financial and Performance Metrics Relate to Inventory?
Inventory metrics translate physical stock into sales, cost, margin, cash, and operational performance. A metric is useful only when its numerator, denominator, period, valuation basis, and scope are clear.
- Cost of goods sold (COGS): The cost assigned to the goods sold during a defined accounting period.
- Gross margin: Sales revenue minus the cost of goods sold.
- Gross margin return on inventory investment (GMROI): A measure that relates gross margin to the average inventory investment used to generate it.
- Inventory value: The monetary value assigned to inventory under the applicable accounting and valuation method.
- Average inventory: A selected average inventory balance used for analysis over a period.
- Inventory turnover: A measure comparing COGS with average inventory to indicate how frequently inventory is converted through sales.
- Days inventory outstanding (DIO): An estimate of the number of days represented by the inventory balance relative to COGS.
- Sell-through rate: The proportion of available units sold during a defined period.
- Stock-to-sales ratio: A comparison of inventory value or units with sales value or units.
- Carrying cost: The cost of holding inventory, including storage, financing, insurance, handling, risk, and depreciation where applicable.
- Working capital: The difference between current assets and current liabilities, with inventory representing part of the cash tied up in operations.
- Inventory accuracy: The degree to which recorded inventory matches physical inventory.
- Cycle counting: A recurring method of counting selected inventory rather than waiting for one complete physical count.
- Shrinkage: The difference between recorded inventory and physical inventory caused by loss, damage, error, or unauthorized removal.
- Forecast accuracy: The degree to which forecast demand matches observed demand under a stated measurement method.
- Mean absolute percentage error (MAPE): A forecast-error measure based on the absolute percentage difference between forecast and actual demand, subject to limitations when actual demand is zero or very small.
- Bias: A repeated tendency for forecasts to overstate or understate actual demand.
- Inventory health: A combined view of stock availability, age, demand coverage, excess, obsolescence, and financial exposure.
- Weekly sales, stock and intake (WSSI): A planning view that organizes expected sales, receipts, stock, and inventory value by week.
- Warehouse management system (WMS): A system that records warehouse execution, including receiving, putaway, picking, packing, transfers, and location control.
How do WSSI, WMS, and ERP Fit Together?
WSSI is a planning view, while WMS and ERP are systems; each serves a different planning or execution purpose. A WSSI organizes expected sales, receipts, stock, and inventory value across time; a WMS records warehouse locations and movements; an ERP connects purchasing, finance, sales, and supply transactions.
Weekly sales, stock and intake, or WSSI, is a planning view rather than a universal software product. Its usefulness depends on consistent inputs such as opening stock, planned sales, intake, closing stock, markdowns, and value assumptions.
An ERP provides cross-functional records for purchase orders, sales orders, invoices, suppliers, and inventory valuation. A WMS focuses on warehouse execution, including receiving, putaway, picking, packing, transfers, and location control. An API or scheduled data exchange may connect these systems, but the exact integration design depends on the platforms and operating model.
Illustrative example:
At a specialty apparel retailer, the merchandise planning team reviews a weekly WSSI before the autumn launch. The sales view shows strong expected demand for one jacket, but the warehouse view shows that several cartons are still in receiving and are not yet available for customer orders. A planner who reads only the purchase order report treats the incoming units as ready stock. The allocation team then sends too much inventory to one store and leaves another location exposed to an avoidable shortage.
The team separates the records. The ERP shows the open purchase order and financial commitment. The WMS shows the cartons in receiving, the units on quality hold, and the units already available for allocation. The WSSI shows the expected sales, intake, closing stock, and value by week. The revised decision delays one allocation, directs available units to the locations with confirmed demand, and records the held stock separately.
The issue was not a lack of data; it was a failure to distinguish planned supply, physical stock, and available stock. Inventory planning becomes easier to learn when each term is tied to a decision: forecast demand, position supply, classify stock, measure financial exposure, or execute movement. Use this glossary as a shared reference when reviewing planning documents, ERP records, WMS activity, or WSSI reports.
What Should a Beginner Learn First About Inventory Planning?
A beginner should first understand SKU, demand forecast, lead time, on-hand inventory, available inventory, reorder point, safety stock, purchase order, stockout, sell-through rate, and inventory turnover. These terms connect the basic flow from expected demand to supply decisions and financial results.
Next, distinguish planning from execution. A forecast, WSSI, replenishment policy, or purchase plan expresses an intended decision; a goods receipt, stock transfer, cycle count, or WMS transaction records operational activity. Keeping those categories separate helps teams interpret inventory data without confusing a plan with a completed event.
