An external ID is a custom field that has the external ID attribute enabled, which means it can be used as an identifier when importing and updating records from an external system. It allows administrators to match records based on a unique ID value from another system instead of using Salesforce record IDs, which may not be available or consistent across systems. An auto-number field is a custom field that automatically assigns a unique numeric value to each record, but it cannot be used as an identifier when importing and updating records from an external system because it is generated by Salesforce and may not match with the external system’s IDs. A rich text field is a custom field that allows users to enter formatted text, images, and links, but it cannot be used as an identifier when importing and updating records from an external system because it is not unique or consistent across systems. A record ID is an internal ID assigned by Salesforce to each record, but it cannot be used as an identifier when importing and updating records from an external system because it may not be available or consistent across systems. References: https://help.salesforce.com/s/articleView?id=sf.custom_field_attributes.htm &type=5