A longlist is the full set of potentially suitable suppliers; a shortlist is the reduced set actually invited to submit a proposal.
Building the longlist. Incumbent suppliers, market research, recommendations, prior sourcing events — typically 10 to 30 suppliers.
Reduction. Category experience, capacity, geographic coverage, financial standing, compliance.
Shortlist size. Three to five suppliers; beyond that, proposal quality falls even as competition looks broader.
Documentation. Record why a supplier didn't make the shortlist.