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

Results fortestingz.keys.openpgp.org

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2025-10-03T01:29:07.168	2025-10-03T00:30:36	2026-01-01T00:30:35	testingz.keys.openpgp.org	testingz.keys.openpgp.org	C=US, O=Let's Encrypt, CN=E8
2025-10-03T01:29:06.814	2025-10-03T00:30:36	2026-01-01T00:30:35	testingz.keys.openpgp.org	testingz.keys.openpgp.org	C=US, O=Let's Encrypt, CN=E8
2025-08-04T15:14:30.508	2025-08-04T14:16:00	2025-11-02T14:15:59	testingz.keys.openpgp.org	testingz.keys.openpgp.org	C=US, O=Let's Encrypt, CN=E5
2025-08-04T15:14:30.299	2025-08-04T14:16:00	2025-11-02T14:15:59	testingz.keys.openpgp.org	testingz.keys.openpgp.org	C=US, O=Let's Encrypt, CN=E5