The Service Contract Header Table
This field contains the number of the customer to whom you will send the invoice that belongs to the service contract. The service invoice is considered to belong to a service contract which number is contained in the Contract No. field on the invoice header.
All service contracts and contract quotes contain two customer numbers, one for the customer with whom you have signed the service contract (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 and statistics will refer to the customer who will receive the invoice (Bill-to Customer) and not the one who owns the service items (Customer).