The MathObjectsAnswerCheckers.html file is a very early document that I wrote when I first developed the MathObjects library, and it hasn't been updated very well as changes have been made. So it is pretty our of date. (It's not actually WRONG, but sadly incomplete.) The documentation situation is pretty poor all around.
It should have been in
but seems to have been left out (none of the Formula flags are listed). I'm afraid I usually just look at the code when someone asks me what the flags are. :-)