[Mike Pearson]

Here is my take.
Class racers have a specific set of rules that you have to follow. Body type, engine type, weight, tire size etc. no delay boxes. HC start. Set indexes and qualified order

Bracket racers have mostly only safety rules to follow. Any body, engine and weight. TS and TD might have a min weight. delay boxes and power adders. Random pairing

Index racers have weight and index rules that have to be followed. Any engine, body, specific weights, throttle stops delay boxes. Heads up start. Random pair round one. set indexes

Pro and Semi Pro also have specific rules that have to be followed. engine size and chassis type, weight, fuel type, pro tree

Stock, SS and Comp are class racers
TS and TD are bracket racers
SG,SC and S/St are index racers
Pro Mod, TAD and TAFC are semi pro (technically Sportsman)
TF,FC, Pro Stock, Pro stock MC are professional classes