External ID in Salesforce

The External ID field allows you to store unique record IDs from an external system,typically for integration purposes.

If we create External Id field, it will be indexed by default by salesforce.

During upsert operation

1. If External Ids are matched, it will update the records.
2. If External Ids are not matched, it will create a new record.
3. If External Ids are matched more than once,it will throw an error.

The fields with below data types can only be external Id

1.Number
2.Text
3.Email

You can designate up to 25 External ID fields per object.

External Ids are set with the unique property so that the IDs will be unique to each roecord.

Note :
Unique fields are not used in the UPSERT . it determine the uniqueness.