Per Diem does not show up in IRS rules it comes from the GSA and the IRS recognizes the rates and compensation. Here is the definition of Per Diem from the GSA:
Per diem allowance—The per diem allowance (also referred to as subsistence allowance) is a daily payment instead of reimbursement for actual expenses for lodging, meals, and related incidental expenses. The per diem allowance is separate from transportation expenses and other miscellaneous expenses. The per diem allowance covers all charges, including taxes and service charges where applicable for:
(a) Lodging. Includes expenses for overnight sleeping facilities, baths, personal use of the room during daytime, telephone access fee, and service charges for fans, air conditioners, heaters and fires furnished in the room when such charges are not included in the room rate. Lodging does not include accommodations on airplanes, trains, buses, or ships. Such cost is included in the transportation cost and is not considered a lodging expense.
(b) Meals. Expenses for breakfast, lunch, dinner and related tips and taxes (specifically excluded are alcoholic beverage and entertainment expenses, and any expenses incurred for other persons).
(c) Incidental expenses.
(1) Fees and tips given to porters, baggage carriers, bellhops, hotel maids, stewards or stewardesses and others on ships, and hotel servants in foreign countries;
(2) Transportation between places of lodging or business and places where meals are taken, if suitable meals can be obtained at the TDY site; and
(3) Mailing cost associated with filing travel vouchers and payment of Government sponsored charge card billings.