Number: 11 (coarse) 43 (fine)
This is a percentage of Volume (ie, as set by Volume Controller). In other words, Expression divides the current volume into 16,384 steps (or 128 if 8-bit instead of 14-bit resolution is used). Volume Controller is used to set the overall volume of the entire musical part (on a given channel), whereas Expression is used for doing crescendos and decrescendos. By having both a master Volume and sub-Volume (ie, Expression), it makes possible to do crescendos and decrescendos without having to do algebraic calculations to maintain the relative balance between instruments. When Expression is at 100% (ie, the maximum of 0x3FFF), then the volume represents the true setting of Volume Controller. Lower values of Expression begin to subtract from the volume. When Expression is 0% (ie, 0x0000), then volume is off. When Expression is 50% (ie, 0x1FFF), then the volume is cut in half.
Here's how Expression is typically used. Let's assume only the coarse adjust is used (ie, #11) and therefore only 128 steps are possible. Set the Expression for every MIDI channel to one initial value, for example 100. This gives you some leeway to increase the expression percentage (ie, up to 127 which is 100%) or decrease it. Now, set the channel (ie, instrument) "mix" using Volume Controllers. Maybe you'll want the drums louder than the piano, so the former has a Volume Controller value of 110 whereas the latter has a value of 90, for example. Now if, at some point, you want to drop the volumes of both instruments to half of their current Main Volumes, then send Expression values of 64 (ie, 64 represents a 50% volume percentage since 64 is half of 128 steps). This would result in the drums now having an effective volume of 55 and the piano having an effective volume of 45. If you wanted to drop the volumes to 25% of their current Main Volumes, then send Expression values of 32. This would result in the drums now having an effective volume of approximately 27 and the piano having an effective volume of approximately 22. And yet, you haven't had to change their Volume settings, and therefore still maintain that relative mix between the two instruments. So think of Volume Controllers as being the individual faders upon a mixing console. You set up the instrumental balance (ie, mix) using these values. Then you use Expression Controllers as "group faders", whereby you can increase or decrease the volumes of one or more tracks without upsetting the relative balance between them.
If a MultiTimbral device, then each Part usually has its own Expression level.
14-bit coarse/fine resolution. 0x0000 to 0x3FFF where 0 is minimum effect.
Note: Most all devices ignore the Fine adjust (#43) for Expression, and just implement Coarse adjust (#11) because 14-bit resolution isn't needed for this.