Question:
When firing AA against air units flying GS or DAS missions, does the AA unit fire only against those air units effectively supporting the attack or against all air units in the hex making GS or DAS mission (even those not considered for combat ratio calculations)?

Answer:
AA fire is resolved against all enemy air units flying GS/DAS missions in the hex. After AA fire is resolved, the owning player may decide which air units will participate in the combat resolution, up to the limits imposed by RE considerations.

Source:
TEM 50