Well, internal means that its a part of the turbo as it came from the factory. External would then mean anything else.

I've seen externals that use the stock WG hole in the turbine housing, and I've seen externals that had a flange welded right to the scroll of the turbine (seems to be a neat setup) so its tough to explain the difference since things aren't done traditionally all the time. Basically, its one of those things you just know.