Bigenderboth is a gender in the bigender system in which one has two genders, but how they experience both genders is fluid. One can be both or a mix, but is never either gender separately.
The flag was created by the coiner in the coining post, on the same date. The blue and red represent the different genders that one is fluid between. The bluish and reddish purples represent a mix of the two. The black represents being both separately.