So, the alien shall not be suffered to live in the 41st millennium - this is a fundamental fact of 40k. I get that (and approve, humans are cool.)

But I'm wondering: when exactly does an alien creature become such that it's exterminated on sight? Although corpse starch is a thing, I assume not everyone on every planet is a full time cannibal - that simply wouldn't be feasible in the long term. They would need to supplement with animals and local edible flora - but aren't all creatures that aren't humanity technically xenos? And what about animals used for other things, like mounts and labor?

At what point does a creature go from an "animal" to a xenos, essentially? Is there some kind of designation? Is it simply a matter of the danger it poses to humanity? But that can't be entirely true - because the Tau aren't belligerent the way, say, tyranids are. Is it a combination of sentience and danger? Is there an in universe "litmus test", so to speak?