It's pretty simple really. Heat races are run to set the order of the main. It is all done on your time so most if not all races you are racing against the clock until the main event (IFMAR style). If you run in a large class or at big events it is very likley that there will be A, B, C and so on Main's for each class. The winners of the lower mains bump up into the higher mains.

Not real hard to understand and the track will guide you through the process.

Go out and have some fun !!!