Key is the flag's primary key in the caller's (org, project) store, and the name evaluation looks it up by. On a write it is taken from the URL, never from the body: the stored document's own "key" is forced to match.
UpdatedBy is the email of the principal who last wrote it. Empty when the write came from an in-process composer (an experiment registering its own assignment flag) rather than from a signed-in person.
Version is 1 when the key was created and rises by one on every overwrite. It counts writes, not content changes: re-storing an identical document bumps it.