How do DataSync incremental transfers work?

A customer is planning to use DataSync to copy data to EFS and a question came up about incremental transfers:

If a single byte is changed on a source file after an initial transfer, does DataSync transfer the entire file or just the changed bytes during a subsequent transfer? The documentation states only files that have been added, modified, or deleted are transferred. This suggests to me that an incremental transfer copies an entire file if it has changed, not just a delta of changed bytes. Is that the case or does it work like rsync and transfers only the changed bytes?