What is reference_objects ? (C++)

I want to get all CRhinoObjects which can been seen by me in the 3D view.
Therefore, I implement it by CRhinoObjectIterator.
There is a parameter, reference_objects.
I don’t know it.
This mean that reference_objects isn’t visible ?

enum object_category 
    active_objects = 1,    // objects that are part of current model and saved in file
    reference_objects = 2, // objects that are for reference and not saved in file
    active_and_reference_objects = 3     

Reference objects are objects that referenced from other files, such as worksession models or linked blocks.