If you get paid back for travel costs and want to account for that income separately
First create another revenue account in your Chart of Accounts, call it reimbursements.
Then create "products" for each category of reimbursement using that category:
Then create the invoice with those products, enter descriptions and amounts.
When you get paid and mark this invoice as Paid by reconciling it in your account, the income will go to the Reimbursement account so you can separate it from your taxable income.