It can be anything! An apex, subdomain, or even a URL.
Results fors.windows.microsoft.com.office.zurichairport.myshn.eu
www.s.windows.microsoft.com.office.zurichairport.myshn.euwindows.microsoft.com.office.zurichairport.myshn.eumicrosoft.com.office.zurichairport.myshn.eucom.office.zurichairport.myshn.euoffice.zurichairport.myshn.euzurichairport.myshn.eumyshn.eueu