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

Results forfirebase.api.shopsavvy.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2019-01-24T17:44:08.795	2019-01-24T16:44:07	2019-04-24T16:44:07	hirelogic.app	firebase.api.shopsavvy.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-01-24T17:44:07.998	2019-01-24T16:44:07	2019-04-24T16:44:07	hirelogic.app	firebase.api.shopsavvy.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-01-21T10:34:31.313	2019-01-21T09:34:30	2019-04-21T09:34:30	firebase.api.shopsavvy.com	firebase.api.shopsavvy.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-01-21T10:34:30.225	2019-01-21T09:34:30	2019-04-21T09:34:30	firebase.api.shopsavvy.com	firebase.api.shopsavvy.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3