What is the difference between battlegroup 1 and 2? (Just returned player here)

Sorry guys im just returned and i didnt find any infos about the topic ive mentioned, however im interested about it!

Thanks for the replies :).

Battlegroup 1 = Wotlk (icecrown, lordaeron, blackrock realms)

Battlegroup 2 = Wotlk seasonal (Frostmourne realm and Blackrock characters with "seasonal mode")