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

Results forredirector.airforce

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2019-10-20T17:45:12.182	2019-10-20T16:45:11	2020-01-18T16:45:11	app.road-trip-planner.com	redirector.airforce	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-10-20T17:45:11.891	2019-10-20T16:45:11	2020-01-18T16:45:11	app.road-trip-planner.com	redirector.airforce	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-10-20T17:45:05.224	2019-10-20T16:45:04	2020-01-18T16:45:04	app.road-trip-planner.com	redirector.airforce	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-08-23T07:15:45.749	2019-08-23T06:15:44	2019-11-21T06:15:44	redirector.airforce	redirector.airforce	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-08-23T07:15:44.662	2019-08-23T06:15:44	2019-11-21T06:15:44	redirector.airforce	redirector.airforce	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-08-23T03:48:43.819	2019-08-23T02:48:43	2019-11-21T02:48:43	redirector.airforce	redirector.airforce	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-08-23T03:48:43.103	2019-08-23T02:48:43	2019-11-21T02:48:43	redirector.airforce	redirector.airforce	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3