In accordance with the migration to the new upgraded billing system,  

 (a) the primary contact has now renamed as the account owner. 
 (b) the sub contact with access has now renamed as the users. 
 (c) the sub contact without access has now renamed as the contacts. 

Here's the elaboration of each naming using:
(a) Account Owner: (or client account) own products and services. They represent a billable party, like a business, and associated users can access and manage them.
(b) Users: can access and manage one or more of their associated accounts. Separate controls manage user access to each account.
(c) Contacts: The contacts can be added to receive selected billing account email notifications. They are not able to access and manage the account.

Remark: It is important to ensure your account details is valid from time to time. Your password is required to be updated every 6 months due to security reasons. 

Account Owner
Each account can only have a single account owner. Account owner have full permissions to the billing account and can send invitations from the account to another users (by choosing all/certain permissions) in order to get assistance in accessing and managing the account.

How to update account owner details? Refer HERE

How to add/remove user? Refer HERE

How to add/remove contact? Refer HERE