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One of the Touch Response functions.
With this, the keyboard generates MIDI messages in repsonse to additional pressure applied to notes (after they are played). These Aftertouch messages can be used to change the sound in various ways.
There are two types of MIDI message regarding Aftertouch: Channel Aftertouch (Channel Pressure) and Polyphonic Aftertouch (Polyphonic Key Pressure).
Channel Aftertouch generates a MIDI message for the entire keyboard. Polyphonic Aftertouch generates a MIDI message for each note on the keyboard.