A contact can be linked to more than one customer and a customer can, presumably be linked to more than one contact.
Contacts::__pkContactID = Contact_Customer::_fkContactID
Customers::__pkCustomerID = Contact_Customer:_fkCustomerID
Customers::__pkCustomerID = Sectors::_fkCustomerID
Fields with names starting with __pk should be auto-entered serial number fields. Those starting with _fk should be number fields. A portal to Sectors on your Customers layout can be used to list each sector assigned to that customer.
It possible to set up a portal to customers on the Contacts layout to look like a set of check boxes (tick boxes) where clicking an empty box creates a related record in Contact_Customer and clicking a selected check box deletes such a record. If you create one Sector record for each sector that is appropriate for a given customer, a portal to sectors on the same layout will list all the sectors for all the selected customers.
Getting the portal to look and function like a set of checkboxes requires scripting and conditional formatting. Here's a demo file that illustrates this approach as well as some simpler ways work with a many to many relationship: https://www.dropbox.com/s/oyir7cs0yxmbn6i/ManyToManywDemoWExtras.fp7