curious about ukba reference number format

[arsenal49]

My friend made a psw application about 1.5 years ago and she was given ukba ref number in the following format
PBXXXXXXXXXXX

I made a tier 2 app not so long ago and was given reference number in the format PPXXXXXXXXXX

I am just curious what does PP stand for because PB presumably stands for "Points-Based"

Its not a big deal but just slightly curious!

thanks

PS.ignore number of XXX's; its irrelevant to my query

[arsenal49]

i guess its a mystery!