Seigengender is a xenogender that can be described as being confined to a small space. Despite being limited to such a small space, or box, it desperately wants to break out of this box, but can't due to inexplainable reason(s). This constant desire to break out of this "box" with no relief can make a seigengendered person feel discomfort and/or dysphoria. This gender can be expressed in any way, shape, or form a seigengender user feels is natural.
An alternative name for this gender is claustrogender.
The word "seigen" (or 制限) comes from the Japanese word for "limit", and the prefix "claustro" was borrowed from Latin. The use of the prefix was inspired by the word "claustrophobia".
This term was coined by anonymous FANDOM user on the 11th of July, 2021. All three flags were coined on the same day.
(Original: LGBTA Wiki)