External ID Mapping
External ID mapping is used only for Salesforce objects to map references to other objects in target connection and for determining whether to create a new record or update an existing one for the UPSERT operation. External ID in Salesforce is a custom field that has the “External ID” attribute and uniquely identifies records. For External ID mapping, we need to have the External ID field values of the object, the target object refers to, in the source data.
For External ID mapping, click the Column list and select External ID. Then select the source key column in the Source Column drop-down list and the corresponding target External ID column in the External ID Column drop-down list.