If you are catching Rail car or non-stop train from Lahore to Rawalpindi, Lower AC coach will be air-conditioned coach with seating arrangements like a bus. 2 seats on one side and walking area in between and 3 seats on another side.
The trains which are coming from Karachi like Awam Express have different AC Lower coaches. There are several 6 berth compartments (non-covered without door). This is similar to economy class coach, just the difference is Air condition.
Sometimes, when there is no bus type AC Lower coach available, they put the 6 berth compartment style coach in Rail cars. So it depends on your luck which coach is used that day.
This is similar to bus style AC Lower coach but it is much more clean and comfortable. And it comes with the facility of TV and headphones (just like Daewoo Bus).
One of the my pic from 2010 when I came from Rawalpindi to Lahore on Rawal Express:
They are 2 or 4 berth compartment with door (without AC). However, they are not being used now except for Lahore - Quetta route.
They are air conditioned 6 berth compartment coach. Each compartment can be locked by closing the door. It is very comfortable. However if you want to travel on it then it will be available on trains coming from Karachi or Quetta like Tezgam or Khyber Mail or Jaffar Express.
One of my pics from 2013 while traveling from Karachi to Multan on Khyber Mail:
This is the most luxurious coach in Pakistan Railways. It is air conditioned 2 or 4 berth compartment with facility of attached washroom. The sitting space is very spacious and the berths are also wider than usual size. Its also available of long route trains like Tezgam, Khyber Mail or Jaffar Express.
One of the pics from my journey from Lahore to Rawalpindi on Quetta Express in 2004. Back then it was very good, now this coach has become even better: