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

Results forwww.id.trung.dev

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2021-06-01T19:17:43.399	2021-06-01T18:17:42	2021-08-30T18:17:42	www.lab.trung.dev	www.id.trung.dev	C=US, O=Let's Encrypt, CN=R3
2021-06-01T19:17:43.08	2021-06-01T18:17:42	2021-08-30T18:17:42	www.lab.trung.dev	www.id.trung.dev	C=US, O=Let's Encrypt, CN=R3
2021-04-01T17:12:34.624	2021-04-01T16:12:34	2021-06-30T16:12:34	www.ctf.trung.dev	www.id.trung.dev	C=US, O=Let's Encrypt, CN=R3
2021-04-01T17:12:34.467	2021-04-01T16:12:34	2021-06-30T16:12:34	www.ctf.trung.dev	www.id.trung.dev	C=US, O=Let's Encrypt, CN=R3
2021-01-30T17:00:14.319	2021-01-30T16:00:13	2021-04-30T16:00:13	www.lab.trung.dev	www.id.trung.dev	C=US, O=Let's Encrypt, CN=R3
2021-01-30T17:00:13.871	2021-01-30T16:00:13	2021-04-30T16:00:13	www.lab.trung.dev	www.id.trung.dev	C=US, O=Let's Encrypt, CN=R3
2020-11-30T16:17:01.428	2020-11-30T15:17:01	2021-02-28T15:17:01	www.img.trung.dev	www.id.trung.dev	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-11-30T16:17:01.224	2020-11-30T15:17:01	2021-02-28T15:17:01	www.img.trung.dev	www.id.trung.dev	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3