If you are paid a per-diem travel allowance, and it exceeds the amount you spent out of pocket on the business trip, you are liable for taxes on that amount.
If you have to report expenses to your organization, you do NOT report it to the IRS. If not reported, it's declared and actual expensed deducted on your tax return.
If you are required to produce receipts for your expenses, it is not income and not claimed as income. If it is income, it will be included in your W-2.