The Dark Yin Palace was a Gold-tier force that limited its recruitment requirements to females, preferably young and impressionable. But like most sects, they were divided into a series of complex networks of clans, interests, and status. Not without that standard complexity, the Dark Yin Palace was not only composed of female cultivators; they only recruited females.
The distinction was clear, as males born from members of the sect or married into it via the clans, later joining due to that special connection, were also a part of the Dark Yin Palace.
The sect itself established its foundation on its unique cultivation called the Dark Yin Spirit Method, designed to weaponize the Primal Yin quintessential energies and mould a Spirit of Cultivation with its qualities. During the Qi Condensation Realm's Infused Spirituality Phase, the female cultivator would merge their Primal Yin quintessence with their Spirit of Cultivation, establishing a Yin Natal Soul.