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

Results forgitlab01.tools.idc.jp

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2018-07-18T02:58:57.295	2018-07-18T01:58:56	2018-10-16T01:58:56	gitlab01.tools.idc.jp	gitlab01.tools.idc.jp	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-07-18T02:58:56.737	2018-07-18T01:58:56	2018-10-16T01:58:56	gitlab01.tools.idc.jp	gitlab01.tools.idc.jp	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-04-27T03:14:52.646	2018-04-27T02:14:52	2018-07-26T02:14:52	gitlab01.tools.idc.jp	gitlab01.tools.idc.jp	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-04-27T03:14:52.398	2018-04-27T02:14:52	2018-07-26T02:14:52	gitlab01.tools.idc.jp	gitlab01.tools.idc.jp	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2018-02-14T08:10:33.093	2018-02-14T07:10:33	2018-05-15T07:10:33	gitlab01.tools.idc.jp	gitlab01.tools.idc.jp	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2017-12-06T06:38:14.629	2017-12-06T05:33:59	2018-03-06T05:33:59	gitlab01.tools.idc.jp	gitlab01.tools.idc.jp	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3
2017-09-17T06:46:43.68	2017-09-17T05:47:00	2017-12-16T05:47:00	gitlab01.tools.idc.jp	gitlab01.tools.idc.jp	C=US, O=Let's Encrypt, CN=Let's Encrypt Authority X3