X-gender as well as the other terms in the Culture-exclusive Genders folder are genders that are specific to certain cultures and should not be used by people not of that culture. If you're not sure you're able to use this, feel free to send an ask on ask-pride-color-, I have followers that could help you out!
"X-gender is a gender that is usually attributed to Japan, although concepts similar to this exist in most other countries that recognize more than 2 genders (I will explain a bit more later in this submission). So in Japan, X is a third option to F and M, and is seen as an equivalent to saying nonbinary/genderqueer in general. In the case of trans people who wish to denote thier transition, I’m sure most of you know of FTM and MTF. Using X for nonbinary identities (agender, bigender, genderfluid, etc etc), there is also the option of describing as FTX and MTX. One could even say XTX, which would be used for someone who is intersex or just does not want to acknowledge their assigned gender/sex, who identifies as nonbinary. Likewise, there are some people to use XTF or XTM, again if they are intersex or don’t want to reveal their assigned gender/sex, and denote that they are transitioning to man or woman.
I mentioned earlier, the concept of X-gender is usually creditted to Japan, but it is not exclusive to just Japan (so personally, I do not think it is very fair to say it is a gender exclusive to just one culture, although I think perhaps X-gender at least originated from Japan). In some countries, like AUstralia, Germany, and the UK, they have the option of allow people to identify on documents as X instead of F or M as their gender, same in Japan. If you look up X gender, I am sure you will find much more countries that have this option for X.
For this flag, it was designed to be very simple, while also associating with the nonbinary colors (yellow, purple, black, and white).
For other links, I am afraid that there isn’t much in English, other than how countries other than Japan have X as a nonbinary option. Here, here, and here are a few bits of info that some people reading this may not have seen already."
term coined/flag made by Pride-Flags on deviantart @tigerfoox #freetoedit