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

Results forroot.nuconic.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2020-01-10T09:19:20.922	2020-01-10T08:19:20	2020-04-09T08:19:20	nuconic.com	*.nuconic.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-01-10T09:19:20.762	2020-01-10T08:19:20	2020-04-09T08:19:20	nuconic.com	*.nuconic.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-11-11T08:51:47.404	2019-11-11T07:51:46	2020-02-09T07:51:46	nuconic.com	*.nuconic.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-11-11T08:51:46.797	2019-11-11T07:51:46	2020-02-09T07:51:46	nuconic.com	*.nuconic.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-09-11T00:56:48.969	2019-09-10T23:56:48	2019-12-09T23:56:48	nuconic.com	*.nuconic.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-09-11T00:56:48.276	2019-09-10T23:56:48	2019-12-09T23:56:48	nuconic.com	*.nuconic.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3