There was a discussion about that in the STCC-forum.
To my understanding the three classes will be decided upon after round 1 (Heat A, B & C) has finished. After that the possible pace of the cars should be visible from the times.
But I want to remember the clerk of the course that after Round 1 only 8 people will be on the server and the clerk has to call all drivers for participation in Round 2 individually (via LFS spectator), as the status of backmarker and midfield class is not so clear (and the drivers are new to the format anyway).
I imagin something along these lines: 7 GMT, clerk calls for Heat A and the cars, Round 1 Heat A starts, after that the clerk calls for Heat B and names the car, Round 1 Heat B starts, the clerk calls for Heat C and announces the car, Round 1 Heat C starts. The clerk calls for Round 2 Heat A and calls the names of the drivers who are to join, Round 2 Heat A starts, after that the clerk calls for Round 2 Heat B and announces the drivers for this session, Round 2 Heat B starts, the clerk calls for drivers of the feature race and the feature race starts.
This is rather complicated, but otherwise it will end in mayhem because some drivers will definitely be misinformed about the status of their car and which heat to join at what time.
And to the drivers: Remember that you are supposed to listen to the ingame-chat via
LFS spectator
Vain