Research->Computer Music->Knowledge
This page provides general information about control change, RPN, NRPN, and System Exclusive.
Operations of Sound Generator 
There are three kinds of command to operate Sound Generator. They are Control Change, RPN and NRPN, and System Exclusive Commands. Using these command makes the MIDI file more sophisticated. 
Control Change 
Control Change Commands are used to change the settings of volume, modulation wheel , Pedals, and so on. Control Change Commands contains operations of Synthesizer's switches, and users can change these settings from MIDI. 
RPN and NRPN commands have similar functions with Control Change Command, although they are newer. RPN and NRPN enables changing tuning, changing music instruments, and so on while the file is playing. RPN stands for Registered Parameter Number and NRPN stands for Non Registered Number. RPN can be used with all Sound Generator, while NRPN can be used with only specified maker's Sound Generator. 
System Exclusive 
System Exclusive includes all commands that you can operate with your Sound Generator. In other words, Control Change, RPN, NRPN are subsets of System Exclusive. However, Control Change, RPN are common among all makers, so use these commands and avoid using System Exclusive when it is available. 
Difference between System Exclusive and NRPN 
When you want to use the maker's original functions, you use System Exclusive Commands. However, System Exclusive Commands are intended to use for setting at the beginning of the tune, and it is not suited for real time setting and channel setting. So, NRPN are provided to change tones and channel setting when the tune is played.