The doctors' specialties.

I just got to season 2, and I do not understand how their specialties work. Can any of you explain to me, who does what? I mean, that other medical show (yes the 20++ seasons long) made it clear that one surgeon has one specialty, mayne two if you're double boarded. but this show, I'm currently watching Dr. Lim, a trauma surgeon, operating on a patient's brain for aneurysm clipping. Is she double boarded (trauma + neuro)?