Any flight plan uploaded to the downloads section can be used as a race course. When you create a race for a flight plan it gets designated as a Race Course and shows up as a course card in the race course carousel on the racing page.
You can think of a race course as the same as a Formula 1 or MotoGP race course. For example, Silverstone Circuit in Northamptonshire, UK is the home of the MotoGP British Grand Prix motorcycle race, but that is hardly the only race run on the track. It hosts many races for a variety of classes of both cars and motorcycles. A simflightplan race course is the same, you can create races of varying laps and difficulties targeted at a particular class of aircraft.
Once a flight plan has been used as a course, it will be available as a selection in Cockpit Map whenever you want to create a new race using that same route. Courses (flight plans) can be laid out any way you like, either in a closed-loop arrangement for flying laps like the Paris Circuit, or a timed run in a single direction, like a timed run down the Grand Canyon.