The number of documents that matched the filter.
The number of documents that were modified.
OptionalupsertedThe _id field of the upserted document if an upsert occurred.
It MUST be possible to discern between a BSON Null upserted ID value and this field being unset. If necessary, drivers MAY add a didUpsert boolean field to differentiate between these two cases.
Determines if the upsert did include an _id, which includes the case of the _id being null.