Minor Section 10.3.1.2 Differential Update uses the term "component" to specify how a PATCH request must be processed, without stating whether a "component" is only a direct property of an entity type (treating complex properties as atomic), or also a property of *(a property of) a complex type.
The latter makes more sense as the server must support PATCH on directly addressed complex properties, and forcing the client to construct a batch request that first updates the primitive properties of an entity, then in subsequent requests the simple properties of complex properties, and so on recursively seems to be a waste of resources.