http://www.dragunov.net/optic_pso1.html
I would like to point the dev team to this resource that describes this sight wonderfully.
TL;DR: PSO has a battery slot, which allows it to illuminate the entire reticle in red color (somewhat akin to red dot scope). Also, it has an IR filter, which shows infrared light emitters (primitive night vision).
Wishful thinking: Once you guys have the new renderer working, hopefully you will make PiP scopes, and PSO will be one of them.