Each transaction should have a type defined when adding sales receipts and sales receipt refunds.
The list of the available sales receipt types can be edited on the Financial → Settings → Sales Receipt Types page:
All types are available to be selected for sales receipts transactions:
The settings used for online payments are described in the following article: Online Payments: Features and Settings.
Irrelevant types can be marked as Old, and they will be discarded from the list of available types for new payments.