Originally Posted by Zorak

Broadly speaking, any change to a ticket that results in an additional amount needing to be collected due to higher fare and/or change fee.

Notably, if you use a gift card to pay for part of a ticket, it seems to get processed in such a way that any remainder is treated as a/c, and if you pay that remainder with your Plat card... there are a few data points in the thread of amounts below $200 being reimbursed.