External ID matching for upserts
Key Takeaway
An upsert matches each row on an external ID: a match updates the existing record, and no match creates a new one. In the Data Import Wizard, an external ID field is offered as a match option only when it is also Unique.
An external ID field stores each record's ID from a system outside Salesforce. When you upsert data, the import compares the external ID values in your file with existing records. Because of that matching, you can load data keyed by another system's IDs without creating duplicate records. Data Loader and the Data Import Wizard both match on external IDs, but each tool has its own rules.
What an external ID is
Your company links Oracle Financials with Salesforce. You create an external ID field on Account. Each account record now captures and displays its Oracle ID.
External ID fields can be text, number, email, or auto-number. An auto-number external ID isn't available for matching. You can still use an auto-number external ID to look up the parent record if the parent record contains the external ID.