Technically, you can do whatever the hell you want. Your license will say 'unrestricted medicine and surgery,' but what keeps the IM doc from performing Neurosurgery is ethics, malpractice, and privileges (as far as I know). If you wanted to do something like become boarded in EM and then run some type of primary care clinic/urgent care clinic while you're not doing hospitals shifts, I really don't see how you'd run into much trouble at all. Probably really wouldn't do too much malpractice wise, but you CAN'T advertise yourself as a BC FM doc. You can say 'BC physician,' 'EM specialist practicing FM,' 'FM expert,' etc, but I think this isn't too uncommon, especially in more rural areas.