SagePay Repeats
From WHMCS Documentation
SagePay Repeats is the remote storage version of the SagePay module where card details are stored on SagePay's server for rebilling rather than in your WHMCS database.
By using the Repeats option the PCI Compliance requirements you have to meet are lessened, and the risks in accepting and storing credit card details on your server are completely negated.
The client's card details are sent to SagePay upon their first payment and stored and can be charged at a later date. To update their credit card details the client would need to pay an invoice using the new card details and they will be updated at SagePay at the same time.