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

Results forbackend2.planemo.io

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2020-10-10T17:46:37.018	2020-10-10T16:46:36	2021-01-08T16:46:36	backend2.planemo.io	backend2.planemo.io	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-10-10T17:46:36.797	2020-10-10T16:46:36	2021-01-08T16:46:36	backend2.planemo.io	backend2.planemo.io	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-08-11T09:16:08.007	2020-08-11T08:16:07	2020-11-09T08:16:07	backend2.planemo.io	backend2.planemo.io	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-08-11T09:16:07.865	2020-08-11T08:16:07	2020-11-09T08:16:07	backend2.planemo.io	backend2.planemo.io	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-06-12T05:10:15.286	2020-06-12T04:10:14	2020-09-10T04:10:14	backend2.planemo.io	backend2.planemo.io	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-06-12T05:10:14.773	2020-06-12T04:10:14	2020-09-10T04:10:14	backend2.planemo.io	backend2.planemo.io	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-04-12T19:05:04.483	2020-04-12T18:05:03	2020-07-11T18:05:03	backend2.planemo.io	backend2.planemo.io	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-04-12T19:05:04.148	2020-04-12T18:05:03	2020-07-11T18:05:03	backend2.planemo.io	backend2.planemo.io	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3