That means they're blocking the hex for any other aircraft units that want to come in to hit the same target in the same turn.
Suggestion- give aircraft a free 1-hex movement "hop" after bombing/strafing so that they can hop to an adjacent hex to allow a second aircraft unit to come in, then a third and fourth etc in dramatic realistic "waves"..

(The Order of Battle game uses the "1-hex hop" feature and it works magnificently.)

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