Indirect lookup relationship in Salesforce.

Use an indirect lookup relationship when the external data doesn’t include Salesforce record IDs.
An indirect lookup relationship links a child external object to a parent standard or custom object. When you create an indirect lookup relationship field on an external object, you specify the parent object field and the child object field to match against each other.
Specifically, you select a custom unique, external ID field on the parent object to match against the child’s indirect lookup relationship field, whose values are determined by the specified External Column Name.
If the external system uses case-sensitive values in the specified External Column Name, make sure that the parent object field is also case-sensitive. When you define the parent object’s custom field, select External ID, Unique, and Treat "ABC" and "abc" as different values (case sensitive).