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 to 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.
If you set a parent concept on the physical table, and the parent concept and the physical table both have the same property, the physical table does not recognize the parent concept's value for that property. This is because the physical table already has the property as part of its self concept, and the self concept always overrides the parent concept. Since you cannot remove a required property, the parent concept's property will never be recognized at the physical level.
To override a physical object's required property, set a parent concept at the business model or business view level. Since you can override the inherited properties, the parent concept at the business level takes priority.
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.