Aircraft are split into classes typically grouped by similar performance characteristics. The race classes from the MSFS Reno Air Races are standard, as are several classes for aircraft that don’t meet the criteria of any of the official racing classes, such as commercial jets.
There are no restrictions in simflightplan racing, you can race any class aircraft in any race, however only laptimes from aircraft in the race class assigned to that race will be counted as official. Each simflightplan pilot can see his or her own laps in all cases, but if the aircraft flown was not in the race class for the race, those laps won’t be counted in the official results. For example, the Paris Circuit T-6 Open is a 3 lap race using the Paris Circuit and is restricted to T-6 Texan style aircraft. There are also several other races along the Paris Circuit for Unlimited Class and Light Propeller Class aircraft. You can fly any of them in any aircraft, but only laps in T-6 aircraft will be considered for the official race results for the Paris Circuit T-6 Open race.
The aircraft class is detected automatically when you load an aircraft in MSFS, so if it doesn’t match the aircraft class for the race you want to join, Flightplan will ask you if you want to add the aircraft to the class. It is not necessary to add your aircraft to the class – you can race regardless – but if you purchased an aircraft in the Marketplace for example, and it hasn’t been raced on simflightplan.com yet you can add it to the appropriate class(es).
An aircraft can exist in more than one class and the classes are regulated by simflightplan.com moderators. You can create your own custom class if you want a very narrow set of flight characteristics allowed in your race. Classes can be assigned a Max Airspeed and an Airspeed Floor which can be used to restrict the aircraft that can join the class. For example, the T-6 class is restricted to a Maximum Airspeed of 250 MPH (speeds are always in MPH for racing) so you should not join a P-51 Mustang into the T-6 class because its cruise speed is higher than 250 MPH. You can also restrict the lower end using the Airspeed Floor, if you set this at 250 MPH (for example) aircraft slower than that will not be allowed in the class. So you shouldn’t join an X-Cub in a class with an Airspeed Floor of 250 MPH because it’s cruise speed is around 140 MPH.
Because of the diversity of aircraft and the fact that you can even customize and rename your aircraft in MSFS, Flightplan will not restrict which classes you can join. However moderators will delete your aircraft out of a class if it obviously doesn’t meet the characteristics of the rest of the aircraft in the class. Please do not join a MiG-29 into the Light Propeller Class, it will get deleted by a website moderator. As mentioned above, you can race any aircraft in any race without joining the class, so if you just want to race your MiG-29 on a Light Propeller Class race, go ahead and do it without joining the class. If you then decide if you want to create a new Jet Class race, just create a new race with the same aircraft loaded and your new race will automatically be in the Jet Class in this example.