See these answers: https://www.odoo.com/forum/help-1/question/customer-portal-licences-33308

And also the: ODOO ENTERPRISE SUBSCRIPTION AGREEMENT V4.1

2. Definitions

2.1 Customer

Is considered as customer, the signing party to the Agreement (referred to as “Customer”), having his Odoo instance or database hosted on his infrastructure – on his premise or not – or hosted by a third party.

2.2 Users

A user (“User” or “Users”) is defined as an employee, a customer, or a vendor of the Customer who has or will access Odoo Apps in create and/or edit mode. External users such as customers of Customer or vendors of Customer which are declared as portal users are not counted in the number of Users. For the avoidance of doubt, a number of examples follow:

• an employee of Customer who reports his/her expenses or time sheets is counted as a user;
• a third party accountant who uses Odoo is counted as a user;
• customers and vendors which are declared through the portal will not be counted as users;

Clients of Customer using the Odoo website will not be counted as users.

Retrieved on 11.02.2016 from: https://www.odoo.com/page/odoo-enterprise-sla-eu