NERC is not a member of the RFL. There are some international events as well.(Brazillian, Austrailian, UK, Korean[I think], and I know of an event being planned in India)
Most US events are part of the RFL for the sake of simplicity and making it easier for bots built for one event to compete in another. NERC has simmilar rules to the RFL, with the addition of the "Sportsman" class, which bans spinning weapons over a certain speed and weight as well as wedges and robots with no active weapons.
There may be more small events not under the RFL, but I can't think of any at the moment.