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

Results forprometheus.stage1.code.cool

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2018-12-30T13:03:37.26	2018-12-30T12:03:36	2019-03-30T12:03:36	prometheus.stage1.code.cool	prometheus.stage1.code.cool	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-12-30T13:03:36.911	2018-12-30T12:03:36	2019-03-30T12:03:36	prometheus.stage1.code.cool	prometheus.stage1.code.cool	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-12-30T12:31:33.713	2018-12-30T11:31:32	2019-03-30T11:31:32	prometheus.stage1.code.cool	prometheus.stage1.code.cool	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-12-30T12:31:32.735	2018-12-30T11:31:32	2019-03-30T11:31:32	prometheus.stage1.code.cool	prometheus.stage1.code.cool	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3