All physical and some business objects in the metadata model have a set of required properties. These properties are set automatically on creation of the object, and are not removable (although you can change their value). The purpose for required properties is disallow users from getting into a predicament where they have removed a property that is integral to the SQL generation process. For example, if a physical table did not have a Target Table property set, the SQL generator will cause errors, because it cannot access the correct physical table to query.
Using the previous example, when you set a parent concept on the physical table, and the parent concept has a Target Table property, the physical table does not recognize the parent concept's value for Target Table. This is because the physical table already has a Target Table property as part of it's self concept, and the self concept always overrides the parent concept. And since you cannot remove a required property, the parent concept's Target Table will never be recognized at the physical level.
So how do you override a physical object's required property? You set a parent concept at the business model or business view level. Since the inherited properties are override-able, the parent concept at the business level wins.
All physical metadata objects, the business categories and the business model have required properties. You can see what's required for each by referring to Required Properties per Business Object.