In the past I asked people to do an experiment: Create a blank performance, pick any sample for element 0 (use no other elements), set the sample pan to full R (63), and then in the common settings set the pan to full left (-64 or -63). The sound vanishes. You can hear it getting quieter and quieter as you pan left.
Normally you would expect that the sample just ends up back at the center and sound fully normal, as this is indeed what happens on most synthesizers. Instead it vanishes.

Here is why this happens: The sample is “baked” into the L and R channels, and the common pan works by modifying the levels of the L and R channels after this.
So when you pan fully right, the sample itself generates nothing in the left channel and 100% volume on the right channel. Then when you use common-pan, this panning simply amplifies or reduces the volumes of the current L and R channels to create a pan effect. When you common-pan fully left, the L channel goes to 100%, but the sample generated nothing on the left channel, so it produces no output. On the other hand, the sample generated full output on the R channel, but since the common pan has been shifted fully left it reduces the R channel to full silence. The final result is full silence.

This is useful to know, as it will explain why samples with pans (drums, detailed pianos, etc.) always have unexpected volume levels when the common pan (which is controlled by MIDI) is anywhere but center. IE, if your samples have pans, keep the main pan for that element at center as a rule of thumb. If you need to shift its panning, it would be better to break the sample into different elements with centered sample pans and adjusted common pans.

Another example of where this would be necessary to know is if you are recording something like a jazz song and you want a crisp clean detailed piano sound off to the left. Your result will not be as expected if you are using a detailed stereo piano performance, as parts of the piano will drop in volume disproportionately. On the other hand, if the piano only has centered sample pans, it would work as expected to pan the whole piano to the left.

Keep this in mind and happy composing.

L. Spiro