The Service Contract Header Table
This field contains the number of the customer who owns the service items in the service contract/contract quote.
All service contracts and contract quotes contain two customer numbers, one for the customer who owns the items in service (in the Customer No. field), and another for the customer to whom you will send the invoices that belong to the service contract (in the Bill-to Customer No. field).
All customer-related information regarding invoicing, discounts, statistics and so on will refer to the customer who will receive the invoice, not the one who owns the service item.