An AIX PID is composed of the process table slot number and a generation count. Depending on whether you use a 32-Bit or a 64-Bit kernel the format differs slightly. In a 32-Bit Kernel it is a 32-Bit number and a 64-Bit number in the 64-Bit Kernel. However in both cases only the first 26 Bits are being used actually. E.g.
Bit 0 is always zero, therefore every Process ID except for init is an even number.
The genereation count is used to prevent PID being used again to often. I.e. every process slot can be used 128 times before a formerly used PID is reused.
The process table slot index is the process table slot number.
The remaining bits are unused.
If you see a PID with an uneven number you found a Thread ID.
SRAD stands for Scheduler Resource Affinity Domain. Those bits are used to select the zone of the process table. The number of SRAD bits is version/release dependent. 5.1 uses 5 bits, 5.2 and 5.3 use 4 bits.