Approval Routing for Business Unit Access Changes in Purchase Agreements

When Business Unit Access details are modified on an approved agreement—whether CPA or BPA—the Purchase Agreement is automatically approved according to the existing rules defined for Purchase Agreement Change Orders. However, based on the business requirement, any changes to Business Unit Access should trigger an approval workflow.
Currently, I’m unable to identify any attribute that governs approval routing based on changes made either on the Controls page or within the Business Unit Access section. Could you please confirm if such an attribute exists or suggest an alternative approach to enforce approval in these scenarios?