Let's assume you're receiving a RIS from a unit that has both primary and secondary radar. The controller calls traffic to you and intimates, "no height information." This could mean the traffic is an aircraft that has no transponder, has a transponder that isn't switched on, has a transponder which has no mode C capability or has a transponder with mode C capability but with the mode C deselected (for some reason, good or bad). If the unit is providing RIS based on SSR only the service should be limited for that reason because all they'll see are the squawkers. Keep those eyes peeled for the traffic the controller cannot "see".