@michmoor I believe that is for clients IPs.. I don't currently have squid or squid reports or anything installed, guess I could to take a look. But anything you google for squid PTR all comes up talking about the client IP.
from back in the day, when I ran proxies for living ;) we almost always blocked direct IP access, and only specific ones were whitelisted. Not sure why a proxy would want to look up PTRs when you normally block direct IP access, etc. ;)
But for clients, you could use client names in rules that allow, deny etc. So since client IPs might change you might want to do ptr on client IPs to know if its specific client based on its name.