It can be anything! An apex, subdomain, or even a URL.
Results fors.windows.microsoft.com.portal.o365proxy.mvcpoc.myshn.eu
www.s.windows.microsoft.com.portal.o365proxy.mvcpoc.myshn.euwindows.microsoft.com.portal.o365proxy.mvcpoc.myshn.eumicrosoft.com.portal.o365proxy.mvcpoc.myshn.eucom.portal.o365proxy.mvcpoc.myshn.euportal.o365proxy.mvcpoc.myshn.euo365proxy.mvcpoc.myshn.eumvcpoc.myshn.eumyshn.eueu