They're typically submachinegun sized, but firing a round designed to penetrate armour and still provide adequate terminal effects from a pistol caliber sized cartridge.
The KAC PDW is a slightly different take, just a miniaturized piston AR, firing a round optimized for very short barrels and high controllability while having superior projectile energy at range than 5.56 from a similar length barrel.
I think .300BLK and 6.8SPC SBRs might be a more universally recognized system encroaching on the KAC PDW's niche, but with less controllability in mind.
*edit: My SBR and PDW, a Gen3 WE open bolt
