G-Suite Domain is part of Google Cloud Platform with some legacy baggage. (2017-08-12)
G-Suite Domain customers can associate their email accounts with an DNS Domain name. When you do this, each email account takes the form username@yourdomain.com. You can specify an identity by using any DNS Domain name that is associated with a G-Suite Domain account.
Like groups, G-Suite Domain cannot be used to establish identity, but they enable convenient permission management.
The G-Suite Domain "G-Suite Super Admin" are the Google Organization Owners