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

Results forwebdisk.ebook.zootpoint.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2019-06-03T11:18:42.314	2019-06-03T10:18:41	2019-09-01T10:18:41	ebook.zootpoint.com	webdisk.ebook.zootpoint.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-06-03T11:18:41.82	2019-06-03T10:18:41	2019-09-01T10:18:41	ebook.zootpoint.com	webdisk.ebook.zootpoint.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-06-03T06:16:42.206	2019-06-03T05:16:41	2019-09-01T05:16:41	ebook.zootpoint.com	webdisk.ebook.zootpoint.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-06-03T06:16:41.739	2019-06-03T05:16:41	2019-09-01T05:16:41	ebook.zootpoint.com	webdisk.ebook.zootpoint.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3