The A not and B not channels are the compliments of the A and B channels. This means that when signal A is high, signal A not is low and when A is low, signal A not is high. The same is true for any signal that has a compliment. This is commonly used to keep noise to a minimum. Some input cards will accept both the A and A not signal. It then compares the two signals to help eliminate common mode noise that may have been picked up on the line. This is done by accepting a pulse only when signal A is high and signal A not is low. This is true for any channel that has a compliment, signal A was used only as an example. This is commonly called a Differential Output.