Can indirect lookup relationship fields be created on external objects only?

Indirect lookup relationship fields are specifically designed for external objects to reference records in Salesforce. This type of relationship allows external objects to integrate seamlessly with Salesforce data by linking to a record in Salesforce based on a unique external identifier.

The key aspect of an indirect lookup is that it relies on an external object, which makes it different from standard lookup relationships used for standard or custom objects within Salesforce. Therefore, it is accurate to say that these relationships can only be created on external objects.

All other options address different scenarios that are not applicable:

• Creating them on internal standard or custom objects would not adhere to the definition of indirect lookup relationships.
• The Professional Edition has its own set of limitations not directly related to this particular feature.
• Validation status of external objects does not determine whether or not an indirect lookup relationship can be created.

Thus, the statement that only external objects can have indirect lookup relationship fields is indeed true.