What is 'credit from past subscription'?

'Credit from past subscription' is marked next to a customer's payment when an upgrade/downgrade has been made and credit has been carried across from a previous payment.

The customer payment tab can be found under Customers >> Customer List >> Select customer name >> switch to payment tab.

For more information on the payment and credit, click 'Full Details'

You will now be able to see the total credit that was applied from a past membership, along with various other information on the transaction.

For more information on upgrades/downgrades and why credit is sometimes applied, see: Upgrade or downgrade a membership on behalf of a customer