When customers purchase your classes, appointments, subscriptions or packs, they're given the opportunity to save their card on file for future purchases. This reduces "friction" or steps to completion in future checkouts.
If they choose to save the card on file, this will also make it available when you are ringing them up using Point of Sale, provided you're in a territory that allows for this. In the payment section of a POS sale, you'll then see the card populate as a selectable option.
Note: Momence will not allow an admin or front desk operator to save the card on file for future purchases. This needs to be done by the customer when they are self serving.