Availability guard

The rule exists because of a failure every warehouse recognises. Two customers need the same part in the same week. Whoever prepares their delivery first takes the stock they need, and the earlier delivery discovers on the loading dock that its material is gone. Nothing was stolen and nobody made a mistake; the stock was simply allocated in the order people happened to walk up to the shelf.

The guard turns delivery date into the thing that decides. Before a box can be allocated to today's picking list, the system totals the full ordered quantity of that part on every live list with an earlier delivery date, and refuses the scan if the remaining free stock would drop below that figure. Stock that is blocked, in quarantine or already staged for another truck is excluded from the on-hand side, because none of it is genuinely available.

The detail that matters is that the earlier list reserves its whole requirement, not just the portion it has yet to prepare. A delivery that is half-picked still needs all of its material to ship. With 1 200 pieces free, an earlier list needing 1 000 and a later one trying to take 300, the guard refuses: 1 200 minus 300 is less than 1 000. The picker sees the refusal at the scanner, with the arithmetic behind it, rather than discovering the shortage a week later.