A formal written question from the contractor to the architect or owner, asking for clarification on plans, specs, or scope.
RFIs are how a contractor gets clarification without freelancing an answer. 'The plans show 2x10 floor joists but the structural notes call out 2x12 — which controls?' goes out as an RFI, the architect answers, and now you have written cover.
On commercial jobs RFIs are tracked in a log with response deadlines. A slow RFI response can become the basis for a delay claim.