I need a work around.
Combo Box attributes (peer single relationships in this case) cannot have presentation rules.
I have a Contract that can be sold internally by a person or externally by another company.
I have different BO for People Contacts and Company Contacts and have created a BOG called SalesPeople.
People Contacts have psEmployer relationships, but Company Contacts do not, so that attribute does not show up in the BOG.
The filter became too unmanageable to use a single Sold By field populated with the BOG SalesPeople. So I reverted to putting a switch on the form and a hide rule based on the switch value Internal/External sale which shows the peer relationship for internal staff or external agents.

This works quickly and is tidy.
But now I can't get the Required flag to show because there are no presentation rules on Combo Boxes... The BO is generating the required error on the field that receives the Person or Contact but it just doesn't show correctly in the UI because the fields are being faked.
I could set the both the input fields to be required, but then the form will never save because it can't be both externally sold and internally sold...