Delta column in a table is use to do Incremental/Delta loads.

So the prospective Delta columns in your tables could be audit columns like, UpdateDate/ModifiedDate, timestamp, rowversion, ETL_LoadID, etc.

Using CustomerID would be a bad idea, even if its a IDENTITY PK column, as old customers could also be updated as Delta, not only new records.

Check this video "SSIS with Delta Load" for more details: https://www.youtube.com/watch?v=-Kb7uQTB5Ek