When you create an item sale in the Sales window and click the Reimburse button to open the Customer Reimbursable Expenses window, the field at the top of the window will be labeled Reimbursement Item.
If one or more of your reimbursables is not an item, you can create a default Reimbursement Item for these reimbursables. This "item" will allow you to add the reimbursable expense to an item invoice even though the expense isn't an item. This default "item" isn't a physical item at all; it exists only so you can place any of your reimbursable expenses on item sales.
Step by step
To create a default reimbursement item