Your customer wants to create an association, with the requirement that one division can belong...

To create an association where one division can belong to many business units, you must build a Valid When association in the Division object definition.

Valid When associations define conditions under which an object (e.g., a Division) is valid for another object (e.g., a Business Unit).

This setup supports the business rule that a single division may be linked to multiple business units while ensuring data integrity in the hierarchical structure.