I have always tried to educate residents in what MD stands for federally and locally. Federally, there is no restriction to what you can and can not do when you have MD (or DO) behind your name. It states that after 1 year of training, in most states, you have the right to practice medicine on your own. The government never specifies what you have to practice and what training. That actually gives you the ability to have a transition year and then go out and open a clinic and do whatever you want to. Now, doing something like that would put you at huge risk at law suites and losing you license. However, if you wanted to go and do liposuction after a transition year by opening a private clinic, most states will allow it. As an EM doc, I have received sign out from IM docs, FP, peds, even a plastic surgeon in a couple of the small rural EDs where they just want a body in there to keep it open. I asked the plastic surgeon what he would do if he had to intubate someone. He said he would have respiratory do it. I then asked what if they couldn't get it? Has he ever done a crich or been trained on how to do one? I listened to a deposition of a FP doc who did not have the training for rapid sequence intubation literally cry about how he killed a child because he sat and watched as the child slowly stopped breathing and then couldn't intubate him in the end (he had to wait until he could attempt intubation with versed and things).
Most larger facilities require you to be trained and then later to be board certified in your specialty to reduce risk to the hospital but sometimes these smaller ones don't have a choice and just need warm body in the position. You can dabble all you want in any specialty, the federal government allows it once you have the MD, DO or whatever. But you take on huge liability because they will hold you to the standard of that specialty, not yours.