That's an update from last year, when Facebook expanded from only "male" or "female" to 58 total gender options. Users could select up to ten options at once.
Now, Facebook will offer a free-form field to let people enter whatever they want. You'll still be able to choose up to ten options and control who can see your custom gender.
"We recognize that some people face challenges sharing their true gender identity with others, and this setting gives people the ability to express themselves in an authentic way," Facebook writes.
The expanded custom gender option is only available to US users for now.