Nongender is gender that is most accurately described by what it is not rather than by what it is. Nongenders are used by people who are more certain of what their gender isn’t rather than what it is. For example, someone who is a nonboy is definitely not a boy, but they may not be sure what their gender is. They may be more certain in the fact that they are not a boy in they are about any other fact of their gender. However, a binary girl, who is sure they are a girl, would not need to use the label nonboy, as their identity is rooted in the fact that they are a girl, not in the fact that they are not a boy. It's important to note that if someone is a nongender of a binary gender, they are not necessarily the other binary gender. A nonboy is not necessarily a girl, and a nongirl is not necessarily a boy. Nongenders are for people who are more sure of what their gender isn’t rather than what it is.
Nongender is similar, but not the same as, reccugender. Reccugender is where one is confused about their gender but they definitely know what they're not. Nongender people feel as if the only way to describe their gender is by what they're not.