Often prepayments are related to a certain treatment. An invoice is generated with the amount that is pre-paid. It is mostly used for expensive treatment plans or certain (future) surgeries like eyelid surgery.
Adding money to the digital wallet is not counted as revenue but as a 'credit' and this 'credit' can be used for any treatment by the patient during a recurring/new visit. The patient receives a digital receipt as the amount is not seen as revenue as long as it is stored in the wallet.