The master contract for the loan: conditions to funding, representations, covenants, reserves, cash management, transfer rules, and events of default.
The longer version
Everything the lender is relying on lives here: what must be true at closing, what the borrower must do and not do during the term, what gets escrowed, and what happens when something breaks. The other documents implement what this one promises.
For closers, the conditions precedent section effectively IS the closing checklist. Reading it against the actual checklist at kickoff prevents the late discovery that a required item was never requested.
In Prodeal
Checklist templates map to the conditions precedent, so the request list matches what the documents actually require instead of a generic form.