My firm uses the GSA M&IE (meals and incidental expense) rates for travel-related expense reimbursement issues, which break-down per diem by breakfast/lunch/dinner as well as incidentals (it's a flat $3 per day regardless of locality) - according to the GSA site:

Chapter 300, Part 300-3, in the Federal Travel Regulation (www.gsa.gov/ftr), under Per Diem Allowance, describes Incidental Expenses as: A. Fees and tips given to porters, baggage carriers, bellhops, hotel maids, stewards or stewardesses and others on ships, and hotel servants in foreign countries. B. Transportation between places of lodging or business and places where meals are taken, it suitable meals cannot be obtained at the temporary duty site. C. Mailing cost associated with filing travel vouchers and payment of Government charge card billings.