What is 32P in regards to PC1066 RDRAM?

I see that on a lot of the lower cost 256M Samsung RIMMs being sold on the internet, the memory is specified as 1066 - 32P.  I'm guessing 1066 is the speed of the RAM and the 32P is the latency in ns or something or other.  However, I'm wonder what the "P" after the 32 is.  The slower modules have this specification listed without a P after it, ie just a 40, or a 45 etc.

Thanks