Basically a legion in a legion. Can be made if you're a leader in a legion but will take a hell of a lot to create. Each sub-legion has a different type of legion base (total 1 sub-legion base for each profession which means up to 12 sub-legion bases can be made, each serving a different type of role for the entire legion)
Each type of sub-legion can help build certain special legion modules on your ship(modules only active as long as you're in a legion and that legion has reserached enough to unlock it.
Ex.: You want a good scan or cloak module for your ship, so then go to a hacker sub-legion to ask the players there if they can help build you one.
Note: Legion modules should be made VERY expensive to build so then you may need help to get one.
Role of sub-legion bases: They're smaller than the main legion base and also have their own lvl like the main base. They can serve as barriers with their own attack and defense power to help keep the main base from getting disabled. They have their own resource production available only to members of that sub.
Speaking of amount of members in a sub-legion each sub-legion can expand the overall player # limit of a legion by up to 20+ any limit-expanding modules. (Meaning that its player capacity is 20 as original)
When players join a legion, depending on what profession they are, they auto-join the sub-legion of that profession. If the sub-legion is full though they will join the main legion with its 50-player limit. If THAT's full though then they can't join since the other subs are of different profession.

A player with any profession (even if they're in the main legion) can get both the sub-legion's and the main legion's res production combined (split between all members as usual)

Each sub-legion has it's own research line (which type it is depends on the sub's profession type). Successfully researching a phase in there can allow your sub to build a legion module of their type. Since research requirements would be high in a legion all the members in the sub can contribute to it.
Uh if anyone can tell me all of the professions in the game and what they do then i'll tell more. as of now:
Hackers: cloak
Explorers: scan
Raiders: attack/raid%
Excavators:
Fixers: hull
Builders: Planet buildings?

If you have any questions then post it
Oh, and thank you Lone.Lycan for telling me how many professions there are.

Just for the sake of recruiting, the Vanguard Elite Squadron is still trying to recruit active players. If you're the active sort of version, then plz join!
Base lvl: 2

Say what?

With enough luck, he'll clear everything up tomorrow.

if you are referring to what we currently know of as professions, there's already a total of 12 professions in the game, only each race can only pick from a pre-determined 6 professions...