We like to think of credits like a pantry. You always want to eat the things that expire first and save the ones that expire latest for last - that’s why credits (both brand-specific and Catch Cash) are applied by the earliest expiration date.
Here's an example:
Jenny has 1 Girlfriend credit for $20 expiring on June 1st and 1 Catch Cash for $30 expiring on May 25th.
She adds a pair of socks for $15 to her cart and checkouts with Catch. Since she has enough credits to cover the entire cost, credits from her $30 Catch Cash will be applied first - because they expire the soonest.