What is selectable RF modulation?

Some radios will transmit to different receivers. Hitec can be changed from "positive" shift for Futaba to "negative" for JR and Airtronics. Some receivers can be changed to work with either (FMA Direct) at least in some cases. Most all transmitters use FM to transmit but may also have a PCM (pulse code modulation) that can be used (it also transmits on FM) however the receivers are unique to each brand. Bill