PO3S is a positive space flight deck JS listing. It has nothing to do with the cabin. It can be used for such things as check airmen doing a line check, SOWA (or whatever that acronym is?) audits, FAA inspector, etc. If you are commuting and see that listing, assume that they will show up and be first in line for the JS. If you are secondary on a flight with a PO3S, I'd make alternate plans.
I have seen where PO3S's don't show up, for whatever reason.. but if they do, they will bump a PRI/SEC (depending on how many JS's are on the airplane).