Genuine question, this came up repeatedly in the Scottish consultation. All I could find is someone does not 'identify' as male or female. Transgender/transexual people are going from one gender to the other, often making physical changes whether through surgery or hormones. We've established a few don't but have stereotypical traits of the opposite gender they were assigned. They then want to be viewed as the other binary gender as far as possible, even if they never will be on biological grounds or in spaces women are at their most vulnerable.
Whatever way I could see it, is a non binary simply a man or woman who doesn't accept the stereotypes of either binary?