On hand, allocated and available
A warehouse reports several different quantities for the same product, and people new to the Stock screens usually assume one of them is the real one. They are all real; they answer different questions.
On hand is physical. It is what a person walking the aisle would count.
Allocated is committed. Somewhere in BMS, something has claimed part of that stock: a service activity needs it for a visit, a sales order has promised it to a customer, a dispatch has it staged to go out. The stock has not moved, and it is not yours to give away.
Available is on hand minus allocated — the only number you can safely promise from.
Projected is where the product lands once everything already in motion has happened: incoming receipts add, outgoing dispatches subtract.
Why not just decrement on hand?
Because the commitment and the movement happen at different times, and sometimes days apart. A part is claimed for a visit on Monday and leaves the building on Thursday. If claiming it decremented on hand, Monday's warehouse count would disagree with the shelf, and a stocktake would report a variance that is not one. If claiming it did nothing at all, two people would promise the same part to two customers.
Allocation is the record of a promise. On hand is the record of a fact. Keeping them apart is what lets the count be trustworthy and the promise be safe.
Where this bites
Promising from the wrong number. On Hand Qty is the largest and most prominent figure, and it is the wrong one to sell from. Available Qty is the answer to "can I supply this?".
Service and Stock disagreeing. A service activity showing Backorder Available is reading availability; Stock Allocation Required means nothing has claimed it yet. Both can be true at once — the stock exists and is unclaimed — and dispatching an engineer on the first without doing the second is how a visit fails on arrival. This is the same distinction, seen from the Service side.
Stock in transit. A transfer out of one warehouse and into another is two events with a gap. During that gap the stock is on neither shelf, which is why transfers appear as Expected receipts at the receiving end rather than appearing instantly.
Negative holdings. They happen — stock issued that the system never knew had arrived. The workspace's own stocktake history has counts named "Negative Stock Holdings", which is what fixing them looks like: a count, reconciled, rather than an adjustment that hides the discrepancy.