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

Results forwebmail.stackofdomains.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2018-08-21T12:48:09.851	2018-08-21T11:48:09	2018-11-19T11:48:09	mrmassive.com	webmail.stackofdomains.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-08-21T12:48:09.624	2018-08-21T11:48:09	2018-11-19T11:48:09	mrmassive.com	webmail.stackofdomains.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-06-21T13:20:49.699	2018-06-21T12:20:49	2018-09-19T12:20:49	stackofdomains.com	webmail.stackofdomains.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-06-21T13:20:49.52	2018-06-21T12:20:49	2018-09-19T12:20:49	stackofdomains.com	webmail.stackofdomains.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-04-22T21:33:28.386	2018-04-21T13:02:16	2018-07-20T13:02:16	stackofdomains.com	webmail.stackofdomains.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-04-21T14:02:16.358	2018-04-21T13:02:16	2018-07-20T13:02:16	stackofdomains.com	webmail.stackofdomains.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3