Web searches for this are drowned in pages offering trading courses, and the InteractiveBrokers API doesn't expand at all on the term.
So what are contract trading classes, and what are some examples, in particular for e-minis?
For options, it is all of the same type of option for a particular underlying. For the e-minis, you are referring, all the calls where the underlying is e-minis is a class. Similarly for all the puts.
For IB's API, trading classes act as another layer for the unique identification of securities which otherwise might be ambiguous. On occasion something will trade with the same symbol, expiration, and currency, but hypothetically because of some corporate action it has a different multiplier.
They can be looked up in IB's symbol database.
List of E-Mini S&P (ES) futures options classes here. (Examples: E3C, ES, EW4, E1A, etc.)
If you want to be very specific, the LocalSymbol is a truly unique identifier and prevents other parameters such as expiration from needing to be entered into contract specification.