I can’t help at all with implementation aspects - but yes, introspectable range means you can tell the client (that is changing the parameter) what the acceptable range is. That might be an integer range, or it might be an enumeration of settings (for instance, camera modes are often done this way). The main use of this introspectable range is for a visual client (in ROS1 this was the rqt_reconfigure GUI), can show set the values of slider bars or drop down menus.