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

Results forexternal.nettex.com.au

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2019-09-27T18:44:45.313	2019-09-27T17:44:44	2019-12-26T17:44:44	cizgiyorum.com	external.nettex.com.au	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-09-27T18:44:44.658	2019-09-27T17:44:44	2019-12-26T17:44:44	cizgiyorum.com	external.nettex.com.au	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-07-29T06:44:13.829	2019-07-29T05:44:13	2019-10-27T05:44:13	external.nettex.com.au	external.nettex.com.au	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-07-29T06:44:13.292	2019-07-29T05:44:13	2019-10-27T05:44:13	external.nettex.com.au	external.nettex.com.au	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-07-29T05:07:51.442	2019-07-29T04:07:50	2019-10-27T04:07:50	external.nettex.com.au	external.nettex.com.au	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2019-07-29T05:07:50.86	2019-07-29T04:07:50	2019-10-27T04:07:50	external.nettex.com.au	external.nettex.com.au	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3