"Axis to use as symmetry plane normal" settings

Hi

I have some inquiries about the stated subject. Under the postbuckling shell tutorial of the verification models of structural mechanics module, it is stated to that different symmetry edges are assigned different values of Axis to use as symmetry plane normal. What actually does this option do? I do not manage to find its definition in the Comsol Reference Manual.

I also notice there are two coordinate systems available: global coordinate system and local edge system. What is the local edge system necessary and only present in the shell module but not solid mechanics module?

Thank you.

Regards
Goh