Open class allows you to adjust all the parameters of the quad in the settings.
In this class you may be racing players who have setup their quad to be exactly like their real quad, you may be racing players who have setup their quad to fly totally unrealistically but very very fast.
Open class is unrestricted and allows players to do whatever they like with their settings, because of this your multiplayer ranking will never be affected or changed in an open class race as the race is not just based on player skill.
Open class race times do not count towards the online leader boards - you must race single class if you wish to participate in online leader board racing.