Iprocurement - managerial approver list of values

Hi, we are in 12.2.4 and we want to use the fonctionnality.

However, as the list of values offers all the employees it allows errors in choosing the right one ; a manager.

How can we restrict the list of values?

Example, we have some  preparesr doing requisitions for many cost centers, so there might be a different managerial approver by cost center. How can we restrict the list of
values?

Thanks!