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. In this field, you'll enter the number of an item that serves as a "placeholder item" on your item sales, so you can enter any type of expenses-items or services-on the item sales. This placeholder item isn't a physical item at all; it merely exists so you can place any of your reimbursable expenses on item sales.
To use a placeholder item, you'll first need to create it using windows in the Inventory Command Center.
Step by step
To create a placeholder item