Four ideas carry the whole module. Meet them once, here, and the workflows and reports that follow will read as variations.
Warehouses and balances. Stock lives in warehouses. For every product in every warehouse the system keeps a balance: the quantity on hand right now. Balances are not typed in and never edited — they are the sum of the movements below.
The movement ledger. Every change to a balance is a movement: a dated line saying what moved, in which direction, how much — always with its unit of measure named — at what unit cost, and which document caused it. A goods receipt writes receipts; a waybill writes issues; an adjustment writes its correction. The ledger is the module's memory, and §5.7 shows how to read a single movement closely. Because the ledger remembers everything, questions like "what did we hold on the last day of March?" can be answered exactly, by replaying the movements up to that date.
Valuation: the moving average. Stock is valued at its moving average cost. When goods arrive, their cost folds into the average of what is already there; when goods leave, they leave at the average as it stands — a delivery never changes the average, only a receipt does. A small example: you hold 10 bags at an average of 100 each (value 1,000). Ten more arrive at 120 each. The shelf now holds 20 bags at an average of 110 (value 2,200). Selling five takes out 550 of value, at 110 a bag — not at 100 because they were "the old ones", and not at 120 because they were the newest. One product, one warehouse, one honest average.
Promised, expected, and available. Three quantities stand beside on-hand everywhere in this module. Committed is stock promised to customers on approved sales orders not yet delivered. On order is stock expected on approved purchase orders not yet received. Available to promise is on-hand minus committed — what you can still honestly sell — and projected adds on-order back, showing where you will stand when everything open completes.