Please note that PP is an abrevaition from Portal Price. Putting it next to the room name in HostelSystem is a sign for you that for all the rooms of the same type the price is taken from the pricing settings of that room. For example:
Your Double Bed Private Ensuite room type is mapped with rooms HAB 6 and HAB 8
However in the Default Room column the room HAB is choosen. That means that prices for room HAB 8 will be the same as the prices for the room HAB 6 because they are mapped with the same Double Bed Private Ensuite room type and the HAB 6 was choosen by you as the room which prices are prevailing.
Marking the room as PP will help you to remember which room you have choosen to be the one that gives the prices for the rest of the rooms mapped with the same room type. In case of any doubts you can quickly compare if the prices on the booking portal are the same as the prices of the PP room in HostelSystem.
To mark the room as PP you should clik Manage--> Rooms & Rates and then double-clikc the choosen room to enter its properties
Then you should simply write PP next to the room name and click Save&Close