Genderfrithen is a form of genderfluidity in which someone is only fluid between genders that are in some way connected to masculinity or femininity, but they are never completely male or completely female. A genderfrithen individual never experiences genders that are completely abinary (removed from masculinity or femininity). They are also never a binary man or binary woman. For example, a genderfrithen individual could experience genders like androgyne, demiboy/demigirl, proxvir/juxera, libramasculine/librafeminine, etc. They would never experience genders like binary man, binary woman, neutrois, maverique, agender, etc.
The first apparent mention of genderfrithen is on Tumblr user Gender-Resource's April 2020 Gender Jargon list. Coined on the blog UncommonGenders via Tumblr.
The genderfrithen flag uses the colors representing male and female from other genderfluid flags, but hueshifted to purple to represent never being a binary male/female.
Another flag was created by user strbwryfemme. @strwbryfemme's flags should be used with caution as fawn has a history of exclusionism against m-spec lesbians. 
Demifrithen is a gender which is partially static and partially fluid. The fluid part of the gender fluctuates between identities which are connected to masculinity or femininity, but it is never completely male or completely female. The static part of the gender is undefined, and can be anything, so it varies from individual to individual.