Hey @Robm, assuming the unique registration number is present in both data sources, you could use the Merge node’s Remove Key Matches mode like so to list all items present in data source A, but not (yet) in data source B:
Example Workflow
You would then add the items to data source B if this wouldn’t just be an example.
A similar scenario (though slightly more complex as it syncs in two ways) would be described in more detail in this recent blog post:
Hope this helps! Let me know if you run into any trouble here