In the credit limit workflow process validation on business units for the approver.

In the credit limit workflow process, how do I update the business units that are checked during the process in order to verify the approver's business unit?  For example, approver 1234 is in address book BU 'AA'. The user submits the customer record for the credit limit change and that customer is in address book 'CC'. The current logic of the credit limit check does not include business unit 'BB'. Where can we see this coding in order to update it adding 'AA' as a check.