Just a reminder that we accept pull requests to the documentation
At a bare minimum if you spot things that are not documented please list them here 
Thanks all!
Please use the tools made available to you (see above) and try to do your best to help us help everyone with as positive and constructive tone as you can muster. Thanks @psb777
You will be able to see I do occasionally submit small documentation amendments. If no one else does so I will submit a pull request for this preprocessor directive tomorrow but I will have to guess what it does, what its precise effect is, and the reasons why one might use it. Such an imprecise (but genuinely best efforts) contribution from me might be useful perhaps if it annoys someone to say what the truth of the matter is. But this approach is not ideal, and may also be considered provocative. As may this.
If you, @BDub, know what the directive does, why it should be used, then please document it. Thank you.