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][1].

List of E-Mini S&P (ES) futures options classes [here][2]. (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. 

  [1]: https://www.interactivebrokers.com/en/index.php?f=463
  [2]: https://misc.interactivebrokers.com/cstools/contract_info/v3.10/index.php?action=Futures%20Option%20Chain&entityId=a27615988&lang=en&wlId=IB&showEntities=Y