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

Results forfacebook.comet.software

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2020-02-19T05:12:09.87	2020-02-19T04:12:09	2020-05-19T04:12:09	facebook.comet.software	facebook.comet.software	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-02-19T05:12:09.652	2020-02-19T04:12:09	2020-05-19T04:12:09	facebook.comet.software	facebook.comet.software	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-12-11T05:12:08.812	2019-12-11T04:12:08	2020-03-10T04:12:08	facebook.comet.software	facebook.comet.software	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-12-11T05:12:08.591	2019-12-11T04:12:08	2020-03-10T04:12:08	facebook.comet.software	facebook.comet.software	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-09-22T18:54:45.526	2019-09-22T17:54:44	2019-12-21T17:54:44	facebook.comet.software	facebook.comet.software	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-09-22T18:54:44.745	2019-09-22T17:54:44	2019-12-21T17:54:44	facebook.comet.software	facebook.comet.software	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3