Runs again with the same idempotency key, which it passes on: the world recognises it.
Runs again as an idempotent one does, given the digest of its idempotency key (decision 0123).
Runs again, and may act twice.
Never runs again: an activation that finds it with no completion delivers unknown, since it may have acted.
Acts on nothing: a state may drop it or change it while it runs, which interrupts it, and a crash runs it again. It sends to no entity and creates none, since a request is an act on the receiver (decision 0128).
How an operation runs, and what a crash does to it, declared for each kind of operation (decision 0011). It sees the operation, never the state: what it uses is in the operation (decision 0018).