Why categories instead of exact species?

Because a simple tool can't safely tell you how, say, your specific hammer coral will behave next to your specific chalice — that depends on maturity, tank conditions and individual variation. Broad categories give you a genuinely useful starting point without pretending to a precision we don't have.

What's a "sweeper tentacle" and why does it matter?

Some LPS corals — Euphyllia (hammer, torch, frogspawn), Galaxea and similar — extend long, thin tentacles at night that sting anything within reach, well beyond their daytime footprint. This is the single biggest cause of coral-on-coral damage in reef tanks, and the main reason "aggressive/sweeper" LPS need generous clearance.

Are soft corals really dangerous to other corals?

Not physically dangerous the way sweeper tentacles are, but soft corals are well documented as chemical aggressors — some release compounds into the water that can suppress or damage nearby stony coral tissue without any contact at all. SPS corals seem particularly sensitive to this. Good water flow and running carbon both help dilute the effect.

Why do anemones get an "avoid" rating against almost everything?

Anemones aren't sessile the way corals are — they wander until they find a spot they like, and can sting, consume or dislodge whatever they settle against along the way, including fish. That unpredictability is why we can't give anemones a safe rating next to anything sessile; give them their own space and expect to keep an eye on where they roam.