A Gen born and raised on a Genfarm for distribution into the Pen system, to be Killed. Such Gens do not appear human because they've been raised drugged so badly that their brain and mind don't develop a sense of Identity.
Out-Territory (Wild) Gens captured and distributed via the Pen system are "Choice Kills" because they have a sense of personal identity, even when drugged, and can be roused to a sharper sense of fear producing a better Kill experience for the Sime. Likewise Domestic Gens, the children of in-Territory Simes who Establish, have a sense of personal Identity that can't be wholly drugged away.
Thus "Pen Gen" refers to a non-person, an animated body without a personality inside. See Willa in First Channel for a description of the awakening of a Pen Gen into a semblance of personhood. Willa was a genius among Pen Gens. After Unity, even when granted citizenship, most Pen Gens could not be released into society to function on their own as adults. The most hopeless were relegated to the Secret Pens, others remained Wards -- see To Kiss Or To Kill.
(Source: House of Zeor, Ch 4.)
Klyd, "Most of them die in changeover...from the drugs they've been saturated with all their lives. The few who survive are trained to become keepers of the pens...they have little memory of their childhood and very little intelligence. They rarely live as much as ten years after changeover."
Gens often worked at harvesting their own food.(Source: House of Zeor, Ch 4 )'In the far distance, on the slope of a hill behind the pennant building, he saw green-clad workers harvesting grain,Gens raising their own food, breeding stock.'