Class D (Disposable personnel) are test subject. The foundation usually recruit them unwillingly in prison. They are inmates on death row. They're terminated at the end of the month. In time of crisis, (meaning they can't find d class) they can take inmates arrested for heavy crimes or sometimes civilians (refugees).

~ Hope you'll enjoy it !

I inspired myself from the Unity design that can be found here :
https://pbs.twimg.com/media/D0mk-LcUwAAtMPM.jpg