An external ID is a custom field which can be used as a unique identifier in a record. External IDs are mainly used while importing records/ data. When importing records, one among the many fields in those records need to be marked as an external ID (unique identifier).
An important point to note is that only custom fields can be used as External IDs. The fields that can be marked as external IDs are: Text, Number, E-Mail and Auto-Number.
