In the scenario below, a company has a custom object called Sales_Help_Request__c. Each record in the custom objects is unique and belongs to only one person. The developer can match the records in the imported file with an existing record in Salesforce. The match will include both the external Id Text and the auto-generated number fields. The developer can use the custom object’s unique external ID to create and update existing records.
How to Import a Salesforce Custom Object