if the appointment that was purchased with the package credits is then cancelled the credits are not returned to the client's balance. To get the service reverted to the package limit of a particular client you can go to Reports -> Payment Transactions and delete/refund the related transaction. (Refund option is available only if you have SBPay.me Pro integration)
if you relate the package with the recurring service and the client uses the package during the checkout process - currently 1 package credit is used for the whole pack of recurring sessions.