Question:
Is a player required to use AA against air units? Rule 22, page 32 states, ” Air units may undergo enemy antiaircraft fire when they fly certain missions. ” The word may seems to imply that AA is optional.

Answer:
See Rule 22B.2, first bullet: “Total the AA strength eligible to fire at the target air unit’. The word eligible here would seem to indicate it is an all or nothing affair; if you choose to not resolve the AA attack then no AA fires, but if you choose to resolve the AA attack then all “eligible” AA must fire. So it would appear that while making the AA attack is optional, who participates in that attack, if made, is not optional.

Source:
TEM 59/60