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

Results forapi.testing.adloop.co

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2020-08-10T00:07:23.503	2020-08-09T23:07:23	2020-11-07T23:07:23	testing.adloop.co	api.testing.adloop.co	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-08-10T00:07:23.206	2020-08-09T23:07:23	2020-11-07T23:07:23	testing.adloop.co	api.testing.adloop.co	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-06-10T16:38:19.805	2020-06-10T15:38:19	2020-09-08T15:38:19	testing.adloop.co	api.testing.adloop.co	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-06-10T16:38:19.665	2020-06-10T15:38:19	2020-09-08T15:38:19	testing.adloop.co	api.testing.adloop.co	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-04-11T05:57:51.487	2020-04-11T04:57:51	2020-07-10T04:57:51	testing.adloop.co	api.testing.adloop.co	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-04-11T05:57:51.326	2020-04-11T04:57:51	2020-07-10T04:57:51	testing.adloop.co	api.testing.adloop.co	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-02-10T11:02:38.537	2020-02-10T10:02:38	2020-05-10T10:02:38	testing.adloop.co	api.testing.adloop.co	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-02-10T11:02:38.369	2020-02-10T10:02:38	2020-05-10T10:02:38	testing.adloop.co	api.testing.adloop.co	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3