Question about MWS symmetry plane?

In order to be able to setup a symmetry plane what are the requirements?
Must the structure posses inversion symmetry with respect to the symmetry plane used? or just the field should be symmetric with respect to the symmetry plane used? or both are required?
Thanks in advance

How can you have symmetry field if you don't have symmetry structure. I think you should you have symmetry field which means you should have symmetry structure as well.
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