That is correct. They also utilize at least a rifle caliber. Another typical characteristic is that they are (usually) crew served weapons. (gunner, ammo bearer, and just what do we call the guy that gets to carry the tripod?)
Ergo, an MP5, Thompson, M3, etc. are SUB-machine guns, as they are individual weapons and fire a pistol caliber.
As was mentioned, the lines can blur at times.
