Originally Posted by fifty_two

Priority check-in means you board the plane first before general boarding , but in this case it probably means you are in group C or 3 (cant remember if that use letters or numbers)

The fast track is about quick line for security checks and maybe passport controls (if applicable)

These are two different things , fast track is useful and can save you time , while priority boarding is just an extra comfort to board the plane before everybody so you can find space for your luggages