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

Results forstaging.undefined.iptcpu.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2021-01-19T03:57:38.403	2021-01-19T02:57:38	2021-04-19T02:57:38	staging.undefined.iptcpu.com	staging.undefined.iptcpu.com	C=US, O=Let's Encrypt, CN=R3
2021-01-19T03:57:38.155	2021-01-19T02:57:38	2021-04-19T02:57:38	staging.undefined.iptcpu.com	staging.undefined.iptcpu.com	C=US, O=Let's Encrypt, CN=R3
2021-01-04T16:29:12.474	2021-01-04T15:29:12	2021-04-04T15:29:12	staging.undefined.iptcpu.com	staging.undefined.iptcpu.com	C=US, O=Let's Encrypt, CN=R3
2021-01-04T16:29:12.337	2021-01-04T15:29:12	2021-04-04T15:29:12	staging.undefined.iptcpu.com	staging.undefined.iptcpu.com	C=US, O=Let's Encrypt, CN=R3