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

Results forspf.nebu.com

Resolver

Location

TXT

Name	TTL	Value
spf.nebu.com	300	v=spf1 ip4:23.101.16.49 ip4:40.117.156.13 ip4:20.50.43.158 ip4:149.72.249.76 ip4:20.50.226.57 ip4:40.118.64.151 ip4:5.10.21.66 ip4:108.142.122.131 include:sendgrid.net ~all