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

Results forspf0.spf.atom.com

Resolver

Location

TXT

Name	TTL	Value
spf0.spf.atom.com	300	v=spf1 ip4:104.130.122.0/23 ip4:104.130.96.0/28 ip4:109.235.74.222 ip4:141.193.32.0/23 ip4:143.55.224.0/20 ip4:146.20.112.0/26 ip4:146.20.113.0/24 ip4:146.20.191.0/24 ip4:159.112.240.0/20 ip4:159.135.132.128/25 ip4:159.135.140.80/29 ip4:159.135.224.0/20 i p4:161.38.192.0/20 ip4:166.78.68.0/22 include:spf1.spf.atom.com -all