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

Results forold.admin.smartlinkgo.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2021-09-22T20:08:19.014	2021-09-22T19:08:18	2021-12-21T19:08:17	old.admin.smartlinkgo.com	old.admin.smartlinkgo.com	C=US, O=Let's Encrypt, CN=R3
2021-09-22T20:08:18.644	2021-09-22T19:08:18	2021-12-21T19:08:17	old.admin.smartlinkgo.com	old.admin.smartlinkgo.com	C=US, O=Let's Encrypt, CN=R3
2021-01-29T14:07:24.016	2021-01-29T13:07:23	2021-04-29T13:07:23	old.admin.smartlinkgo.com	old.admin.smartlinkgo.com	C=US, O=Let's Encrypt, CN=R3
2021-01-29T14:07:23.817	2021-01-29T13:07:23	2021-04-29T13:07:23	old.admin.smartlinkgo.com	old.admin.smartlinkgo.com	C=US, O=Let's Encrypt, CN=R3