Myth: The Movie Forgot to Explain Its Lore
SPOILER!
The gaps are deliberate, but the mission still has rules
The film leaves Saya's full origin, the organization's history, the wider Chiropteran population and the long-term war almost completely unexplained.
That is not the same thing as having no rules. The movie is very specific about what matters to this mission: Chiropterans can hide as humans, Saya can identify them, they are difficult to kill, and the organization has been doing this long enough to operate inside military space.
Why it matters
Blood gives you operational knowledge instead of encyclopedia knowledge. That difference is the movie.