An external ID is a custom field that has the External ID attribute, meaning that it contains unique record identifiers from a system outside of Salesforce. When you select this option, the Data Import Wizard detects existing records in Salesforce with external IDs that match those values in the import file.
An external ID is a custom field that has the External ID attribute, meaning that it contains unique record identifiers from a system outside of Salesforce. When you select this option, the Data Import Wizard detects existing records in Salesforce with external IDs that match those values in the import file.