Provincial AA will fire at ANY AIRPLANE conducting ANY ATTACK at ANY non-air target. This has been tested extensively back in the day.
And if you don't want to take my word for it, try it yourself; start a game as GER, build up a province with max factories and no AA, play to a point that UK strat-bombs you with a STR-bomber there, save the game, edit in 40 divisions of 4xAA on that province, load the game, and watch what happens to that STR bomber during his not-engaging-the-land-units attack.
Alternately, get to a point where someone hits one of your no-AA land divisions with a CAS strike, save the game, edit in a provincial L10-AA on that province, and see the difference in hourly loss to the CAS before and after.
And if you don't want to take my word for it, try it yourself; start a game as GER, build up a province with max factories and no AA, play to a point that UK strat-bombs you with a STR-bomber there, save the game, edit in 40 divisions of 4xAA on that province, load the game, and watch what happens to that STR bomber during his not-engaging-the-land-units attack.
Alternately, get to a point where someone hits one of your no-AA land divisions with a CAS strike, save the game, edit in a provincial L10-AA on that province, and see the difference in hourly loss to the CAS before and after.
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