Why a build is a bill of material
A machine on a deal is not a line saying "one MFD". It is a build: a named structure listing the device and every component that goes with it. That rigidity is the point.
What the structure buys
Stock can be allocated. A photocopier is not one thing in the warehouse. It is a base unit, a finisher, a cabinet, trays, a starter toner set — held separately, counted separately, and arriving from different places at different times. Allocation happens against components, so a build with no bill of material has nothing to allocate against, and BMS refuses to allocate to one. The Components dot comes first on the build's progress track for exactly that reason.
Partial supply becomes visible. A build whose base unit is on the shelf and whose finisher is on backorder is a real and common situation. Because the build is a list, the system can say which line is holding it up. A single "one MFD" line could only be complete or not.
Costing is real. Deal cost is the sum of what the components actually cost, not an estimate typed against a machine.
The same structure supplies the install. The components that were dispatched are the components the engineer expects to find in the boxes.
Why it is fixed rather than free-text
Because everything downstream reads it. Allocation, backordering, dispatch, invoicing and asset creation all walk the same component list. A free-text description would be readable by a person and useless to all five.
The cost is that someone has to build the bill of material before the machine can move, and that step cannot be skipped by typing harder. That is a genuine overhead, accepted deliberately: the alternative is a deal that looks complete and cannot be supplied.
Where the deal ends and the machine begins
A build finishes twice over. The Order track finishes when the goods have been dispatched and invoiced — the commercial transaction is done. The Asset track finishes when the machine exists as an asset, has been transferred to the customer's ownership, and has had its runup and install jobs completed.
Between those two the object changes identity: it stops being stock the business owns and becomes an asset at a customer site — the same asset that Service will later raise jobs against and order toner for. The deal is where that handover is recorded.