Number: 10 (coarse) 42 (fine)
Where within the stereo field the device's sound will be placed (assuming that it has stereo audio outputs). If a MultiTimbral device, then each Part usually has its own pan position. This is generally when Pan becomes useful, because then you can use Pan, Volume, and Balance controllers to internally mix all of the Parts to the device's stereo outputs.
Pan should effect all notes on the channel, including notes that were triggered prior to the pan message being received, and are still sustaining.
14-bit coarse/fine resolution. 16,384 possible positions, 0x0000 to 0x3FFF where 0x2000 is center position, 0x0000 is hard left, and 0x3FFF is hard right. Some devices only respond to coarse adjust (128 positions) where 64 is center, 0 is hard left, and 127 is hard right.
Note: Most all devices ignore the Fine adjust (#42) for Pan, and just implement Coarse adjust (#10) because 14-bit resolution isn't needed for this.