A payments group's default payment type determines how that payment group is moved between funds or posted to general ledger accounts. For example, when a POS session is cashed up, CashDesk will use the default payment type to determine where to post the payment group's overs and unders.
We recommend using a generic payment type as the default payment type. For example, if you have a 'Cash' payment group, use a generic 'Cash' payment type as its default payment type. That way, any payments inside the payment group are moved or posted in a way that's applicable to all of the payment types inside that payment group.