Single class makes your quad default to a certain set of parameters and applies this to all players in the race.
This is designed to level the playing field so that all players are basically flying a quad of the same speed and agility in the race.
Due to this a race win is down to player skill and not quad settings.
Your multiplayer ranking will be adjusted for winning or loosing in single class races.
Only race times that are done in single class mode will be allowed to be uploaded to the online leader boards.
Single class fixes the values of the three drag sliders and the weight slider. It will allow you to modify values for the lift / downforce, prop power and also the battery size as these don't positively affect the speed of the quad.