Better explanation for mesh groups and root units.

Hi.

I'm using AnyPortrait for my project. I have read all documentation and have found the solutions to almost all my needs. But I still not understand the main goals for mesh groups and root units. What those things have been made for? How do I use them right way and what kind of problems they can solve?

Would be great if you add more detailed explanation of those things to the documentation.

Thank you.