system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40,000] How does the Imperium decide which primitive Xenos deserve pre-emptive extermination and which don't?

Throughout the lore, the Imperium has come across primitive Xenos races and either exterminated them before they became a threat or decided against it.

The Tau are a famous example, saved only by last-minute chance.

How does the Imperium choose?
gpt
It's up the Ordo Xenos, for the most part. They evaluate a species' intelligence, psychic potential, reproduction rate, aggression, toughness and many other parameters. Each xeno race is taken into consideration not just on its own, but also as a potential subject or ally to other xenos or to Chaos. The Ordo Xenos tends to err on the side of caution, with the main consideration behind choosing *not* to exterminate a race being that the resources that would be employed in doing it are needed elsewhere.