[this is not a two way binding question, the question is WHY IS THIS CLASS USED AT RUNTIME WHILE NOT PRESENT IN ANY TEMPLATE]

This Binding class override default Binding behavior.

But how if this class used if I don't find it anywhere but in this file :

public class SettingBindingExtension : Binding
{
    public SettingBindingExtension()
    {
        Initialize();
    }

    public SettingBindingExtension(string path)
        : base(path)
    {
        Initialize();
    }

    private void Initialize()
    {
        Source = Properties.Settings.Default;
        Mode = BindingMode.TwoWay;
    }
}

The Initialize function gets called when the app is launched, but SettingBindingExtension is nowhere to be found elsewhere in the Application.

I would expect this to be used in templates binding statements.