Two kinds of Credits, two rules:
Subscription Credits — no rollover
Plan Credits are good for the billing month they land in. At renewal, whatever's left expires as your plan's full grant arrives fresh. A coupon that grants a plan behaves like the plan.
Top-ups and gifts — never expire
Credits you buy as a top-up, or receive as a straight credit grant, live in a protected bucket that never expires.
Spend order protects you
Your plan's Credits are always spent first, so the bought ones are the last to go. You never lose a purchased Credit to a monthly reset.