When you join any FIA race, you are matched first by SR and then by DR. So the top split lobby would consist of the 20 players with an SR of 91-99 and the highest DR available. When the track first loads, you are listed in order of your DR. So the guys listed at the top of the top split lobby will have 75k DR, and the people below will have less. The next lobby down the DR ranking will be the 2nd split lobby.
Lobbies that are made up of players with SR below 91 can't really be considered top split, 2nd split, or anything really. The points available drops dramatically when your SR drops below 91, so you may as well not even compete unless it's for practice and to increase SR for the next race. Most of us just know which lobby we're in based on the players you see. If you join a race and it's full of A+ drivers and you see the fastest guys in your region at the top, like Nico, Hellzfire, Mcqueen etc. for your region, you know you're in the top split.
To get the best matchmaking, always keep your SR high and you'll avoid getting matched with players below your DR level. The matchmaking in FIA races is what makes them both more challenging and more fun.
In my region, the top split lobby usually consists of all A+ players if it's a popular combo. The 2nd split lobby is usually half A+ and half A, 3rd split is all A. If your SR is low, there will be all kinds of different DR levels in there but you’ll encounter far fewer A and A+ drivers below 91 SR.