What is "under-representation"? How do you measure it, and how do you determine when all "certain groups" (which are what, exactly?) are "represented" proportio

What is "under-representation"? How do you measure it, and how do you determine when all "certain groups" (which are what, exactly?) are "represented" proportionately? These are serious questions. How do we decide: what groups should be represented (what about ugly individuals? people with specific disabilities?), and when everything is all "fair"?