How to leverage External Id and Workbench Smart Lookup feature to load related records while maintaining the referential integrity.
External ID ::
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*.
Salesforce won't treat fields marked as "External ID" as needing to be unique unless the "*Do not allow duplicate values*" flag is also checked.
We can only choose Custom Auto Number, Email, Number and Text Field as external id in Salesforce.
Typical Use Cases ::
Used while updating an External Record imported earlier into Salesforce.
To insert related records from the external system and link them appropriately to its parent (maintaining referential integrity).