For business and tax purposes there are two types of plans, accountable(submitting expenses and getting re-imbursed) and non-accountable(per-diem) Generally per-diem is paid to avoid the accounting paperwork and reporting required in expense re-imbursement of employees, and per-diem cannot exceed cerain IRS limits/restrictions or it is considered taxable income. It can certainly be used as a courtesy or bonus beyond just the concept of paying expenses, but it(or that portion beyond IRS limits)would then be taxable income/wages. Mixing expenses and per-diem can be complicated. Sometimes companies reimburse for travel and lodging but pay per-diem for meals and misc. Generally, unless you have a contract that states otherwise, if all your expenses are being re-imbursed or if travel and living are provided directly by a client, per-diem would not be paid. Companies usually consider that per-diem is paid for providing for travel and living.

