Prefer not to say

Forms have offered this option for as long as there have been forms. Left-handed, right-handed, prefer not to say. It has never needed a reason and it does not need one here.

X means whoever published this considered the question and chose not to answer it. It has to be marked deliberately — nobody is ever assigned an X, and it is never what you get for saying nothing at all.

The standard takes no view on it, and holds no record of who has chosen it.