There are usually multiple groups because there are so many kids at this level- this just gives more kids a shot at medaling (some will also divide groups by age). I think a few competitions will divide A and B as "no axel" and "axel" and you specifically register for those groups, but it is not an official level, so you need to read the competition announcement to know exactly what it means.
Usually you just get to the competition and find out you've been put in group A, and it's just to make the flights more manageable.
Can you post an example of the announcement? I searched a few clubs in different regions and couldn't find any that have you register that way.