It can be anything! An apex, subdomain, or even a URL.
www.1.1365oo2._u.romans.co.uk._spf.smart.ondmarc.com1365oo2._u.romans.co.uk._spf.smart.ondmarc.com_u.romans.co.uk._spf.smart.ondmarc.comromans.co.uk._spf.smart.ondmarc.comco.uk._spf.smart.ondmarc.comuk._spf.smart.ondmarc.com_spf.smart.ondmarc.comsmart.ondmarc.comondmarc.comcom