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

Results forrelay.stdaux.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2021-09-03T08:14:25.228	2021-09-03T07:14:24	2021-12-02T07:14:23	relay.stdaux.com	relay.stdaux.com	C=US, O=Let's Encrypt, CN=R3
2021-09-03T08:14:25.057	2021-09-03T07:14:24	2021-12-02T07:14:23	relay.stdaux.com	relay.stdaux.com	C=US, O=Let's Encrypt, CN=R3
2021-07-04T14:59:24.42	2021-07-04T13:59:23	2021-10-02T13:59:22	relay.stdaux.com	relay.stdaux.com	C=US, O=Let's Encrypt, CN=R3
2021-07-04T14:59:23.786	2021-07-04T13:59:23	2021-10-02T13:59:22	relay.stdaux.com	relay.stdaux.com	C=US, O=Let's Encrypt, CN=R3
2019-03-31T11:49:39.305	2019-03-31T10:49:38	2019-06-29T10:49:38	*.stdaux.com	*.stdaux.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-03-31T11:49:38.43	2019-03-31T10:49:38	2019-06-29T10:49:38	*.stdaux.com	*.stdaux.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3