PDW=Personal Defense Weapon.
The idea behind is that the weapon must provide more offensive
capabilities then a pistol, machine pistol or smg yet smaller then
a AR or carbine. But they are largely placed in the SMG class like
your MP5K, Scorpion,MP7,P90. And though often, they are designed
with the HQ personnel like the clerk, storeman, cook in mind for
their ease of usage, they are hardly seen in their hands and are
more often found in SF troopers hands. The idea is often to place
something that can be easily taught to someone who doesn't operate
weapons often, how to use within a minute and throw that chap into
combat. It sure beats using a pistol due to its ease of aiming,
recoil management and such.
Just my 2cents