Apple Logic Pro 7 - Manual (Page 450)

450 Chapter 13 Event List
Val
Value of the controller. Continuous controllers have a range of 0 to 127. Switch
controllers (#64—#90), transfer only two states; off (val=0) and on (val anything
between 1 and 127).
Aftertouch Events
Aftertouch (or channel pressure) events are generated by a mechanical pressure sensor
beneath the keyboard. The resulting sound modulation affects all notes on that
particular MIDI channel.
Num
This column is empty for aftertouch events, as they only have one data byte.
Val
Strength of the pressure on the keyboard (0 to 127).
Poly Pressure Events
Poly pressure events are generated by mechanical pressure sensors beneath each
individual key. The resulting sound modulation only affects that particular note. Only a
few keyboards offer this capability.
Num
MIDI note.
Val
Strength of the pressure on the key.