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

Results for*.testonly.googleapis.com

Logged At	Not Before	Not After	Common Name	Matching Identities	Issuer Name
2015-02-12T10:47:53.771	2015-02-12T10:37:31	2015-03-14T00:00:00	testonly.google.com	*.testonly.googleapis.com	C=US, O=Google Inc, CN=Google Internet Authority G2
2015-02-12T10:47:53.771	2015-02-12T10:37:31	2015-03-14T00:00:00	testonly.google.com	*.testonly.googleapis.com	C=US, O=Google Inc, CN=Google Internet Authority G2
2015-02-12T10:47:46.278	2015-02-12T10:36:45	2015-03-14T00:00:00	testonly.google.com	*.testonly.googleapis.com	C=US, O=Google Inc, CN=Google Internet Authority G2
2015-02-12T10:47:46.278	2015-02-12T10:36:45	2015-03-14T00:00:00	testonly.google.com	*.testonly.googleapis.com	C=US, O=Google Inc, CN=Google Internet Authority G2
2015-02-11T15:21:02.933	2015-02-11T15:09:27	2015-03-13T00:00:00	testonly.google.com	*.testonly.googleapis.com	C=US, O=Google Inc, CN=Google Internet Authority G2
2015-02-11T15:21:02.933	2015-02-11T15:09:27	2015-03-13T00:00:00	testonly.google.com	*.testonly.googleapis.com	C=US, O=Google Inc, CN=Google Internet Authority G2