An associated contact can be assigned to one or more products, so the customer in question will be implicitly notified via email of every aspect of a commercial nature.
Activating notifications on associated contacts can be done in two ways:
Isolated contacts will receive notifications strictly for the chosen service, and global contacts will receive notifications for all services in the client account, without the need to assign it to a specific service.
Regardless of the number of contacts assigned to a service / multiple services, the account administrator will also continue to receive financial emails for those services.
The complete procedure for setting up the associated contacts can be found in the tutorials section: Enabling associated contacts invoicing.