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

Results forprometheus.call.staging.teamwork.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2019-09-19T14:11:48.043	2019-09-19T13:11:47	2019-12-18T13:11:47	prometheus.call.staging.teamwork.com	prometheus.call.staging.teamwork.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-09-19T14:11:47.21	2019-09-19T13:11:47	2019-12-18T13:11:47	prometheus.call.staging.teamwork.com	prometheus.call.staging.teamwork.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-09-19T14:11:18.209	2019-09-19T13:11:17	2019-12-18T13:11:17	prometheus.call.staging.teamwork.com	prometheus.call.staging.teamwork.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-09-19T14:11:17.574	2019-09-19T13:11:17	2019-12-18T13:11:17	prometheus.call.staging.teamwork.com	prometheus.call.staging.teamwork.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3