It depends -- mostly on whether the vendor (the company receiving the data) is comfortable requiring the source to map some fields.

For low volume cases, we can operate with zero mapping of fields. In those cases, we run every transfer as a full refresh.

If the volumes are higher, then we'll typically ask the source to expose a primary key and last_updated_at timestamp field. In those cases, we run incremental transfers. We use the last_updated_at to figure out what data to transfer, and the primary key to merge it into the destination table without creating dupes.