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. When using External ID mapping, the object, the target object refers to, is determined by its External ID field value, specified 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.