Permanent credits are credits you win when passing a race's finish line, those you gain by selling unused cards at the end of a race or step, but also those you purchased in the store.
Those credits are yours! You can use them at any time, whenever you like.
Caution. These credits are not eternal! When you spend them, they are withdrawn from your permanent credit balance.
Contrary to the credits you receive as a start up bonus every time you join a race or leg, these permanent credits are not lost when the race or leg ends, and can be spent on any race.