Foreign key &-& (flag as a semantic foreign key)

What causes this issue?

A foreign key must be flagged either as a semantic foreign key or as a
screen foreign key. In the case of a screen foreign key, a check is
always made for any input to the screen through the mask processor.
This foreign key is always found with internal tables (without under-
lying database table) and also with foreign keys that are contained
completely within a semantic foreign key. Semantic foreign keys are
part of the data model and are further identified by their dependency
factor and cardinality. Apart from the check on screen entry, they
imply referential integrity conditions.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Please flag this foreign key as semantic foreign key or screen foreign
key.

Error message extract from SAP system. Copyright SAP SE.