As of WD04 the specification treats the existence of type_definition resources and the top-level type_definition resource collection as optional. If your implementation of CAMP does not add any new resources or extend any of the CAMP-defined resources, a Provider is not required to implement any type_definition resources or the top-level type_definition resource collection. Now that every resource references the type_definition resource that defines its structure, we should probably revisit the idea that type_definition resources are optional.