A client can add credit to their personal account which they can later use to pay for services/products.
Simply add the client to POS, click Add Client (bottom left), search for the client on the right side panel and click client's name in Search Results.
Once the client is in POS, click Add Items.
Select the Add To Account option.
On the right side panel enter the amount that they are adding as credit to their account, then click Save.
Close the Add Items To Sale... pop up window, click Receive Payment, select the payment type they're using and Finish Sale.
Their account balance will adjust accordingly, which can be viewed via the Actions menu in their client card.
The Account feature works the opposite way as well, where the client can use their account to pay for their sale even if they don't have credit on the account. The account balance will go into negative, and it can be fixed up at a later date.
To do this, add the client and their services/products to POS, click Receive Payment, choose Account as the payment type, and Finish Sale. Then when they are ready to pay the outstanding amount back, add the client to POS, click Add Items, click Add To Account, enter in the amount they're paying back, click Save, Receive Payment, choose how they are paying, and Finish Sale.