Well, the PID instruction takes a block of 23 integers, and can have both positive and negative values, so I'll go out on a limb, and say that it stores it's data as 16 unsigned integers.
The SLC500 PID PV is scaled 0-16383 (14 bit) and the CV is 0-16383. The integral term is a 32 bit number. Refer to the instruction set reference for more information.