The P stands for Peripheral. There are two types of ports in the Wankel rotary engine, namely the side port and the peripheral port. The pre-Renesis 13b engines had side intake ports (holes in the iron side housings leading into the chamber) and peripheral exhaust ports (holes through the aluminium rotor housings themselves)
By utilising a peri port, the air fuel mixture gets a more direct entry into the chamber as well as having much more "valve" overlap. This is akin to putting a pair of race cams in a 4 banger and which is why they don't idle terribly well.
They're also loud as the intake roar is unmuffled, whilst the exhaust pulses shoot straight out, unmuffled by a turbocharger (most PP's are NA). Brap brap brap...~
Randomly, someone in the UK has built a non-20b 3 rotor using Renesis bits. They've a thread going on in the Mazda Rotary Club forums.
Cheerios~