Perpetual Glen
Just to put this question to rest, this is what we're currently thinking:
There are three departments: Science, Tactical/Security, and Engineering. Each has specialties for both ground and space; medical is the stand-out example of a Science ground specialty. This lets ground medical officers specialize in some other science field in the air, so they can be effective in both places.
You can work to rise in rank and command by sacrificing ground and space skills to buy leadership skills (which are useful in their own rights). Some ranks (above a certain one) require an minimum amount of leadership skills to achieve.
The captain (leader of the group/crew) is the one who steers the ship, so the "conn" specialty has become a number of options within the set of leadership skills that he can pursue.
Hope this clears a few things up.
Admirals command Armadas in high level content.