It can be anything! An apex, subdomain, or even a URL.

Results forspf.fi.net.uk

Resolver

Location

TXT

Name	TTL	Value
spf.fi.net.uk	300	v=spf1 ip4:91.240.224.78 ip6:2a03:ee40:961::78 ip4:91.240.224.82 ip6:2a03:ee40:961::82 ip4:176.74.20.137 ip6:2a00:b980:4:1::137 ip4:194.12.14.166 ip6:2a01:5e40:3:20::166 ip4:176.74.21.80 include:spf.protection.outlook.com -all