This class further specializes CachedTypedWritableReferenceCount to also add a node_ref_count, for the purposes of counting the number of times the object is referenced by a "node", presumably a PandaNode.
This essentially combines the functionality of NodeReferenceCount and CachedTypedWritableReferenceCount, so that a derivative of this object actually has three counters: the standard reference count, the "cache" reference count, and the "node" reference count. Rather than multiply inheriting from the two reference count classes, we inherit only from CachedTypedWritableReferenceCount and simply duplicate the functionality of NodeReferenceCount, to avoid all of the problems associated with multiple inheritance.
The intended design is to use this as a base class for RenderState and TransformState, both of which are held by PandaNodes, and also have caches which are independently maintained. By keeping track of how many nodes hold a pointer to a particular object, we can classify each object into node-referenced, cache-referenced, or other, which is primarily useful for PStats reporting.
As with CachedTypedWritableReferenceCount's cache_ref() and cache_unref(), the new methods node_ref() and node_unref() automatically increment and decrement the primary reference count as well. In this case, however, there does exist a NodePointerTo<> class to maintain the node_ref counters automatically.