Does one simply state they are X, or, is medical documentation required, or, is it one's outward appearance? There are very few persons with ambiguous chromosomes (there are somewhat more people born with congenital adrenal hyperplasia having ambiguous genitalia, but these - or their parents - usually pick one gender or the other - usually based on the chromosomes), so I suspect, medical chromosomal documentation would be very, very rare. Can one just say they are X, like declaring an emotional support animal?