If you are sending a single tiff, only send the required from_number, to_number and file. If the source needs to send a fax in segments, the first request will have segmented=true and segment_completed=false.
The first request will return a fax_uuid. The subsequent requests will have segmented=true with the previously provided fax_uuid. On the last segment send segment_completed=true, with or without a file. The fax_uuid will be the same for all segments. If you do not include the fax_uuid in the subsequent requests, previous segments will not be joined for delivery.