'Non-billable' can be selected when adding billable time. Any non-billable entry will be recorded internally, but will not appear on an Invoice or Prebill. Non-billable entries will not show up on Invoices. 

If you are looking for a solution that allows you to track time without charging your Client, entering your time as 'non-billable' may be perfect for you. You can access the non-billable toggle from any area in Rocket Matter that allows you to capture time.