Which legend abilities create chains that can be reacted to?

Ruling: You can react to all activated abilities (exhaust abilities) and triggered abilities ("when X" or "at the end of your turn" effects). You cannot react to replacement effects (like Zhonya's or Sett legend) which replace another effect instead of triggering separately.

Sequence:

• Each usage of an activated ability (like Vi's recycle) is a separate action that can be reacted to
• Triggered abilities create chains when their trigger condition is met
• Replacement effects do not create chains as they modify existing effects rather than creating new ones

Nuances:

• Sett legend is incorrectly worded and has errata in the FAQ - replacement abilities should not use "when X" language as that is reserved for triggered abilities
• Replacement effects replace another effect and do something else instead, as opposed to triggered abilities which do something additional