It's kind of depends what your using it for. No one is going to do a chromosome test just to see if your id matches. And really anything outside of that is changeable. Surgery hormones ect. It says sex on the ID. But unless you want to do those test each time you check the ID we use it as gender. And even if you do those test. You wouldn't be the first person who lived there whole life as one gender but turned out to have the opposite xx or xy chromosomes without knowing. Its rare but it does happen