It's pretty simple.
Scientists--Running the Show. Some for empirical reasons, some for... less than sane or ethical reasons. The Experiments currently know nothing about them, their memories having been conveniently tampered with.
Stalkers--The stealthy background workers who observe, follow and sometimes hunt the Experiments. They also reset measuring machines around the city, refill food/medical/ammo dumps, release new "monsters" and may or may not screw with the experiments for fun. They must retain secrecy however, if an Experiment sees them, they must be silenced. In order to cope with the jobs given to them competently, they are often altered similarly to Experiments. The price to become a Stalker is fairly steep...
(new)Harvesters-- The personnel who travel out into the world and procure people and animals for experiments. They are known to use loopholes such as acquiring missing persons and M.I.A. soldiers, bringing in fresh corpses of the legally dead for revival, and sometimes people come with them willingly or turn over those they would like to be rid of in their lives. On some occasions they've hired whole mercenary bands promising high pay, leading them, ultimately, to become experiments...
Experiments-- People, sometimes altered and sometimes strange to begin with, tossed into the Pit and studied like animals. They are hunted, poisoned, subjected to harsh conditions and forced to partner up or face consequences that include and are on par with death. Equipped with a device that makes it all possible, called "Ticks" by many Experiments because they look a bit like silver bugs. Fun times abound!
Among the Experiments there's a small hierarchy... and it's less a hierarchy than a set of benchmarks to identify what "batch" they were.
First Gen-- The first wave of Experiments dropped in the Pit. Out of fifty only Luke, Cyrus, Seraphina, Paige, Chistyakov, Satero and Vertigo are surviving.
Second Gen-- Just delivered!!! There's a lot less of them this time around, and with considerably fewer alterations for the most part.
Third Gen-- Only rumored among Scientists. Some excitement about a "dominance circuit" in the new "Biotech Link Modules". Whatever that means.