Implementation details. The definition of CBO deals with the instance variables and all the methods of a class. In VB.NET terms, this means non-Shared variables and Shared & non-Shared methods. Thus, Shared variables (class variables) are not taken into account. On the contrary, all method calls are taken into account, whether Shared or not. This distinction does not seem to make any sense, but we follow the original definition.
If a call is polymorphic in that it is to an Interface method in .NET, this is not taken as a coupling to either the Interface or the classes that implement the interface. If a call is polymorphic in that it is to a method defined in a VB Classic interface class (base class), it’s a coupling to the interface class, but not to any classes that implement the interface. This is a limitation of the implementation, not the definition of CBO.
In this implementation of CBO, when a child class calls its own inherited methods, it is coupled to the parent class where the methods are defined. The original CBO definition does not define if inheritance should be treated in any specific way. Therefore, we follow the definition and treat inheritance as if it was regular coupling.