If we're going to look at previous editions (SR5 specifically), basically a machine pistol is inbetween Pistols and Automatics, and can be fired with Pistols skill in SA mode and Automatics in BF/FA mode. It's also as concealable as a Heavy Pistol, unless you use the folding stock. Range-wise, they had Light Pistol ranges. And unlike an SMG, it can be quick-drawn.
For 6w, it's basically 'BF and higher ammo capacity than most light pistols', and the Concealable holster can only fit pistols/tasers, including machine pistols. Plus the ammo is cheaper than that of SMGs.