Depends on whether the travel is for a government contract or not.

If so, CONUS M&IE rates apply for meals and incidentals, no receipts necessary. Actual lodging expense reimbursed, receipt necessary, must be within CONUS lodging limit for the location. (You can choose to stay at a more expensive place, just like you can choose to eat more expenseive food, but you'll only be reimbursed up to whatever the govt rate is for that location.)

If not a government contract, it's actual expenses with receipts for anything over $25 but CONUS limits don't matter. Still best to stay near the CONUS lodging and M&IE rates so as not to raise eyebrows.