It can be anything! An apex, subdomain, or even a URL.
Results for_p.3.4504fg._u.doyb.us._spf.smart.ondmarc.com
www._p.3.4504fg._u.doyb.us._spf.smart.ondmarc.com3.4504fg._u.doyb.us._spf.smart.ondmarc.com4504fg._u.doyb.us._spf.smart.ondmarc.com_u.doyb.us._spf.smart.ondmarc.comdoyb.us._spf.smart.ondmarc.comus._spf.smart.ondmarc.com_spf.smart.ondmarc.comsmart.ondmarc.comondmarc.comcom