They will reuse the number. Anytime a flight is created from point A to point B, they will use that same number. If BTL102 is used, then that is to go back to YIP. If they go somewhere else, then BTL103 will be created for that route.

For example, AAL245 will always be a flight from Regan (DCA) to Los Angles (LAX) Sometimes, but rarely, that same flight number will be used to another airport in case an emergency landing took place.