Ok let me break down a PRL file as far as I understand them:
Every Providers Cell Towers have a number called a "SID"
Your PRL file has a list of SID's that your provider has permission to use (among other random stuff I dont understand lol)
A prl file begins by looking at your "Home SID" and seeing if that SID is usable at your current location, If it isnt then it begins to go down this list testing each SID in the PRL file until it finds one that works.(this is why your battery dies faster when your roaming)
Now certian SIDs in this list will be set higher or "MORE" (as its written in the PRL) and others just to be normal or "SAME".
Now different PRL files will have diffrent SIDs and how often they are searched by your phone (this is why some work better then others for people)
Things you should know:
There is no "magical" PRL that will let you connect to every tower in the world (but if you find one then please send it to me lol)
A PRL file wont let you connect to towers that your provider isnt allowed to connect to (eg: StraightTalk on Verizon), the tower still requires a Username and password that your phone sends automaticaly.
Here are some photos of what a PRL file looks like:
Hope this helps,
P.S. This is what I have gathered from my own research, this doesnt mean its 100% correct.