A signed proposal carries every line item the integrator will install: the £8,400 worth of in-wall speakers, the £14,200 lighting-control package, the £4,200 7.2-channel processor, the £1,800 worth of CAT6 and conduit, line by line. Specifi already knows which of those line items map to which supplier (because the line items are linked to manufacturer-and-distributor metadata at proposal time, not retroactively at PO time). Clicking "Generate POs" on a signed proposal drafts one PO per supplier, with the right items, the right quantities, and the live cost basis pulled from the connected feed (or from the manual supplier's price list, with the last-verified stamp visible).
The integrator reviews the draft POs, edits any pre-fill the project context did not catch (a substituted product because the supplier is short-stocked, a quantity adjustment because the survey caught a longer cable run than expected), and clicks send. The POs go out to suppliers by email as PDFs.