Stock and costing

Stock is a permanent ledger. Every purchase, sale, return and adjustment appends a movement; nothing is ever edited in place. The quantity on hand you see is the running balance of that ledger.

Weighted average cost

Receiving stock recalculates the average cost across what you already held and what you just bought. Issuing stock takes that average as the cost of the sale and leaves the average untouched. This is what makes the profit report reflect what you actually paid rather than the last price you happened to see.

Adjustments

Use a stock adjustment to correct a physical count. It writes a movement like any other entry, so the correction is visible in the ledger rather than silently changing a number.