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

Results forproftpd.codexsto.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2019-04-06T01:22:51.041	2019-04-06T00:22:50	2019-07-05T00:22:50	proftpd.codexsto.com	proftpd.codexsto.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-04-06T01:22:50.464	2019-04-06T00:22:50	2019-07-05T00:22:50	proftpd.codexsto.com	proftpd.codexsto.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-02-05T01:30:28.175	2019-02-05T00:30:27	2019-05-06T00:30:27	proftpd.codexsto.com	proftpd.codexsto.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-02-05T01:30:27.504	2019-02-05T00:30:27	2019-05-06T00:30:27	proftpd.codexsto.com	proftpd.codexsto.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-12-06T14:21:45.525	2018-12-06T13:21:44	2019-03-06T13:21:44	proftpd.codexsto.com	proftpd.codexsto.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-12-06T14:21:44.33	2018-12-06T13:21:44	2019-03-06T13:21:44	proftpd.codexsto.com	proftpd.codexsto.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3