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

Results forcheckyourheart-staging.happycode.nl

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2019-04-26T18:40:04.81	2019-04-26T17:40:04	2019-07-25T17:40:04	checkyourheart-staging.happycode.nl	checkyourheart-staging.happycode.nl	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-04-26T18:40:04.156	2019-04-26T17:40:04	2019-07-25T17:40:04	checkyourheart-staging.happycode.nl	checkyourheart-staging.happycode.nl	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-02-25T14:26:38.156	2019-02-25T13:26:37	2019-05-26T13:26:37	checkyourheart-staging.happycode.nl	checkyourheart-staging.happycode.nl	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-02-25T14:26:37.154	2019-02-25T13:26:37	2019-05-26T13:26:37	checkyourheart-staging.happycode.nl	checkyourheart-staging.happycode.nl	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3