What does the "Allow IVR control" setting effects if enabled on the Extension Rights tab. The only explanation I can find is that allows IVR control, but which IVR and what kind of control?
"Allow IVR control" feature allows this particular extension to control the IVR calls. It means that this user (extension) can Pick Up, Divert and Park the IVR calls from 3CX Phone for Windows (Presence large screen).