Whether this delta only revises spend already reported for the request.
A correction re-states what an earlier delta already contributed: a completion replacing its chunks, or a request re-priced under the model it finally named. New spend is not a correction, even when it arrives on a request that already has a record, because the tokens themselves are new.
Lets the consumer drop a correction whose subject is gone -- a backend total has already absorbed that request -- whatever the correction's sign. A stale positive correction would otherwise re-inflate a display the total had just settled.
