This is a method for deducting hotel, meal, and incidental expenses on a business trip.
The IRS has divided all the cities in the United States into high cost cities and low cost cities. Then they have assigned two per diem rates, one high and one low. Instead of collecting all those piles of receipts, an employer can just deduct the appropriate per diem rate for his employees’ business trips. The boss and self employed people are not allowed to use this method; they must keep receipts.