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

Results forsos.monitoring.id.test.denic.de

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2019-03-18T13:43:04.59	2019-03-18T12:43:03	2019-06-16T12:43:03	sos.monitoring.id.test.denic.de	sos.monitoring.id.test.denic.de	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-03-18T13:43:04.007	2019-03-18T12:43:03	2019-06-16T12:43:03	sos.monitoring.id.test.denic.de	sos.monitoring.id.test.denic.de	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-03-15T10:46:06.375	2019-03-15T09:46:05	2019-06-13T09:46:05	sos.monitoring.id.test.denic.de	sos.monitoring.id.test.denic.de	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-03-15T10:46:05.718	2019-03-15T09:46:05	2019-06-13T09:46:05	sos.monitoring.id.test.denic.de	sos.monitoring.id.test.denic.de	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3