After you sign a contract or place on order, e.g. on the Virtuosity site you receive a Fulfilment Document. A Customer Number listed in the Order Information section is linked to and identifies your Site, of which all the details are listed in the Site Information section.
Customer number is unique.
In many cases there may be a business requirement to have multiple Sites - they may be for example company branches located in different regions, or even countries. Those branches are otherwise still entities of one company/organization. Each organization has its own identity called an Account Number (sometimes interchangeably called Ultimate or Hierarchy).
Since an organization accommodates Sites, consider the Account Number as a "parent" of a Customer Number - at least one Customer Number needs to be linked to the Account Number, but there may be more.
Entitlements (licenses) are managed by administrators on the Account (Ultimate) level in the Entitlement/License Management portal. If the organization accommodates sites (customer numbers) in various countries, they will be represented as options and available in the Entitlement Country drop-down list.
It is the Account Number (not the Customer Number) displayed in the 'Your Subscription Type' section on the Home page of the Entitlement/License Management portal.
For more details, see: Entitlement Management Overview
Users are managed by administrators on the Account (Ultimate) level in the User Management portal. The Bentley Identity Management System (IMS) manages the Bentley Cloud Services profiles (also called Bentley IMS accounts) that exist for the users in the organization. This means users can only be associated to a single Account and they use their login to access the licensing associated to their account - not to their Site (Customer No).
If an account (hierarchy) accommodates sites in different countries, users are entitled only to those licenses, which were purchased for the country they reside in, and for which they are configured.
For more details, see: Managing Users
Entitlement Management Overview