Underwriter is the lender's credit officer who reviews and approves (or rejects) loan applications. Underwriters work from the program's credit policy and from the case-by-case judgment that experienced files require.
In business-purpose lending, underwriters evaluate: property value (via appraisal), title status, entity vesting, borrower liquidity and experience, rehab budget defensibility (for construction), exit strategy plausibility. They do not evaluate borrower W-2 income for DSCR loans — DSCR uses property cash flow.
Borrowers communicate with underwriters through the loan officer, not directly. When the loan officer says "the underwriter is asking for…," the underwriter has identified a condition that needs to be cleared.