If you haven't already read the Sales Overview, see Entering activities and items for time billing invoices for more information.
Choose the method you wish to use:
To enter activities (automatically) and items on a time billing invoice
To enter activities (manually) and items on a time billing invoice
NOTE: In order to include both activities and items on a time billing invoice, the preference I Include Items on Time Billing Invoices must be marked in the System view of the Preferences window.