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

Results forold.dev.mesasummit.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2020-02-10T07:55:44.124	2020-02-10T06:55:43	2020-05-10T06:55:43	old.dev.mesasummit.com	old.dev.mesasummit.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-02-10T07:55:43.864	2020-02-10T06:55:43	2020-05-10T06:55:43	old.dev.mesasummit.com	old.dev.mesasummit.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-02-10T07:09:40.716	2020-02-10T06:09:40	2020-05-10T06:09:40	www.old.dev.mesasummit.com	www.old.dev.mesasummit.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-02-10T07:09:40.574	2020-02-10T06:09:40	2020-05-10T06:09:40	www.old.dev.mesasummit.com	www.old.dev.mesasummit.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-02-10T06:56:40.502	2020-02-10T05:56:40	2020-05-10T05:56:40	old.dev.mesasummit.com	old.dev.mesasummit.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2020-02-10T06:56:40.303	2020-02-10T05:56:40	2020-05-10T05:56:40	old.dev.mesasummit.com	old.dev.mesasummit.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-10-06T23:35:52.52	2019-10-06T22:35:52	2020-01-04T22:35:52	old.dev.mesasummit.com	old.dev.mesasummit.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-10-06T23:35:52.304	2019-10-06T22:35:52	2020-01-04T22:35:52	old.dev.mesasummit.com	old.dev.mesasummit.com	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3