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

Results fortest1.disroot.org

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2019-11-02T21:54:18.947	2019-11-02T20:54:18	2020-01-31T20:54:18	test1.disroot.org	test1.disroot.org	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-11-02T21:54:18.718	2019-11-02T20:54:18	2020-01-31T20:54:18	test1.disroot.org	test1.disroot.org	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-08-29T15:20:35.77	2019-08-29T14:20:34	2019-11-27T14:20:34	test1.disroot.org	test1.disroot.org	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-08-29T15:20:34.906	2019-08-29T14:20:34	2019-11-27T14:20:34	test1.disroot.org	test1.disroot.org	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-06-29T22:39:01.313	2019-06-29T21:39:01	2019-09-27T21:39:01	test1.disroot.org	test1.disroot.org	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-06-29T22:39:01.081	2019-06-29T21:39:01	2019-09-27T21:39:01	test1.disroot.org	test1.disroot.org	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3