12 Bit AI and 16 Bit AO

It is a resolution.

the no. of bits used to represent an analog signal defines its resolution. The higher no. of bits used, greater will be the accuracy of the quantized signal to its original value.

bit is the binary form where 1 = hi and 0 = lo

This bits are programmed inthe image tables where certain allocation of memory are assigned for AI and AO bits AI and AO module convert analog signal into digital signal which is in form of binary signal and are allocated to the image tables for further processing of the signal this what we see in seimens PLC

hope you get it

Adil