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

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For more information on the payment and credit, click 'Full Details'
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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.
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For more information on upgrades/downgrades and why credit is sometimes applied, see: Upgrade or downgrade a membership on behalf of a customer

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