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Note that IDs in the API calls represent unique objects and the ID field is present in almost all functions. When you are adding a new object (for example, AddCustomer) you should leave the ID unset or set to zero. Updates are made based on this ID, so when modifying an existing object (for example, UpdateCustomer) it is important to set the ID.
Also see SOAP Data Structures for descriptions of the data structures used in the SOAP API calls.