Not that I know a whole lot about the subject, but my impression is that "Internist" is kind of an anachronism in terms of practice. It was the name used to describe MDs who were not surgeons and that took care of patients who were "interned" in the hospital a long time ago. Currently "Internist"' is just a name that doesn't really have to do with the way IM docs practice now a days.
In contemporary practice the specialty has become roughly divided between docs who prefer outpt and those who prefer inpt. The ones that do mostly outpt are the primary care docs and the ones that do mostly inpt are hospitalists. These are unofficial paths that an IM doc can take if he/she chooses not to sub specialize.
That's my understanding.