C++ extern class definition

I'm reading some code that goes:

extern class MyClass : BaseClass
{
    ...
} MyInstance;

Does the extern refer to the class declaration or the instance?

From stackoverflow

• Instance. Classes cannot be extern.

  Although the code smells - this snippet suggests that true declaration of that instance uses a separate class definition. Bad, bad idea - defining the class twice.

  Scott S : Seva nailed it. Separating the instance from the class definition, the extern class does not compile, but extern MyClass MyInstance does.