LEfSe, setting subclass

Dear all,

Hi, i’m working on a microbiome dataset related to Crohn’s disease.
I want to use lefse to analysis the difference of microbiome abundance between classes.

If i set behaviors(symptom) as class and prognosis as subclass, is this make sense?
Or diagnosis age as class and prognosis as subclass?

What’s the purpose of assigning subclass and condition of subclass?