Assuming you are talking about the German AA WTs in HON;
1) Anti-Aircraft units have either the letters AA on their front or blue or green numbers on their to-hit Table. See page 87: Anti-aircraft WTs, with blue or green to hit numbers on their To-Hit table or AA to the front...
2) You will notice that the German 88mm in Heroes of Normandy, Heroes of North Africa and Heroes of the Motherland does not have either. In fact, the letters ATG are printed to the front so it cannot fire at airplane.
2) The 88mm in Heroes in Defiance (HID) is marked AA. Technically, it could fire at airplane, although the stats for the 88mm were first assigned to this WT in HON, in an ATG role thus "balanced" with the various non-German vehicles stats. The airplane stats, and the way they are attacked, is a different "subsystem' and thus, the 88mm does not really fit into this. When it first appeared in Heroes of the Blitzkrieg (HOTB), there was no plane in the module so it did not mattered. When In Defeat Defiance (IDD) came up, planes were introduced for the first time. Fortunately, there is no scenario involving the 88mm against airplane in IDD or HID. Some possible solutions are:
* Put a split to-hit table, such as the one that can be found behind the British 40mm AA gun in HONA, with black and blue numbers.
* Replace the letters AA with ATG at the front of the counter
* Introduce a module scenario rule saying that the 88mm cannot be used in an AA role
all of which cannot go into effect until there is a reprint (and frankly, there is no need to "correct" this at this point).
4) Thus, out of the 2 units you mentionned, only the 20mm can be used against aircraft, of which there is none in HON (there is a plane counter, but it is used only in one event in one scenario, and only to indicate the location of an airstrike that is resolved differently than regular airplane attack because, again, at the time it was designed, there was no airplan standard rules)
