When using a source-based incremental load strategy, the primary key is used to compare the incoming records in the raw table with the existing records in the valid table. An incoming record that does not exist in the valid table based on the primary key comparison will be inserted as a new record. Existing records in the valid table will be updated with the values of the incoming record in the raw table.
See these KB articles for reference:
- Understanding Deployment Strategy for Incremental Tables
- Incremental load method on data source creation level