I think PY refers to Premium Economy, C refers to Business Class, and F refers to First Class.
The type of fare you select when searching can determine which cabin class you are eligible for an upgrade. Mid-range Economy fares may only show PY, but higher-end Economy may show PY and C.
Anyway, here are the messages I've seen and my interpretation of them:
- "[#] person(s) waiting" >> No upgrades are available, but shows the number of people on the waitlist, including if 0 people are on the waitlist.
- "Seats available" >> An upgrade to the relevant class of service is available.
- "Waitlisting" >> I have no idea how this differs from "0 person(s) waiting" which is a version of #1 above