I agree with Guppy - it's more of a proximity call.
Frankly, getting a check 6 too often is way better than getting none at all. I give out as many check6s I could whenever I see an enemy near a friendly.
Another thing is that people give out check6s way too late.
When an enemy plane is already diving down on a friendly, it's usually too late. Someone should have given him a check6 when the moment they saw an enemy plane nearby him. An early warning is needed, so the pre-warned friendly can check the situation, make a judgement, or request others nearby to do a specific action regarding to the bogey nearby.
When he gets a check6 call and sees a bad guy behind him at 600 yards, that's much too late and ineffective.
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So, under that agenda, I give out very many check6 calls. I think probably almost everyone in Rooks has an experience of getting a check6 call from me. I really can't help it if some of those calls, were mistakenly given out.
In most cases, since I don't have a mic, I give out check 6 calls and explain the situation by radio. Also, I give check6 calls to a friendly who is being chased by an enemy to indicate that I have seen the situation, and will try to help: usually I type "keep speed up otw" in most cases.
Personally, I think the one-stroke check6 call is probably one of the best features in the game...