A different servo is required for each control function or radio channel. Again, these systems are generally used in surface vehicles. When shopping for a radio, you will find that people refer to different types of modulation. They are referring to the way the electronic control information is sent from your transmitter to the receiver over radio waves. The control information is transmitted by varying the amplitude of the signal.
Information is transmitted by varying the frequency of the signal.
Using PCM adds additional protection against interference from various sources. This feature allows the modeller to reverse a servo's rotation direction at the flip of a switch. Servos can be mounted in the most convenient way without concern for their rotation direction. The proper movement can then be selected when the installation is completed. The modeller can tailor the sensitivity of his model to his own preferences.