Foreign Key Relation with Field Fixed Constraint

I was wondering why this is not possible. If I add a foreign key relation to a table, the field fixed constraint is not available.

I want to store a reference to two different tables depending on an enum value. For example:

1) if TableA.VehicleType == VehicleType::Sedan then TableA.VehicleRecId = SedanTable.RecId

2) if TableA.VehicleType == VehicleType::Truck then TableA.VehicleRecId == TruckTable.RecId

Do I maybe misunderstand the purpose of a field fixed constraint?

Further I have noticed the following in standard AX implementations:

1) The ParentTableId and ParentRecId method

2) Normal relation with field fixed constraint

Which do cover the solution, but a foreign key with a field fixed constraint would be ideal.

Thanks for reading