What is the meaning of each shape key?

Thank you for this wonderful add-on.

The faces that can be generated using this add-on come with 418 shape keys (blendshapes), but what do the 300 Shape parameters, 100 Expression parameters and 18 Pose parameters mean?

I would like to know if there is any site or video that explains this, as it would be inconvenient to try to tweak all the parameters to see the changes.