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

Results forprometheus1.stage.code.cool

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2018-12-30T01:15:24.092	2018-12-30T00:15:23	2019-03-30T00:15:23	prometheus1.stage.code.cool	prometheus1.stage.code.cool	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-12-30T01:15:23.223	2018-12-30T00:15:23	2019-03-30T00:15:23	prometheus1.stage.code.cool	prometheus1.stage.code.cool	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-12-12T13:59:40.829	2018-12-12T12:59:39	2019-03-12T12:59:39	*.stage.code.cool	*.stage.code.cool	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-12-12T13:59:39.822	2018-12-12T12:59:39	2019-03-12T12:59:39	*.stage.code.cool	*.stage.code.cool	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3