This query was raised with me earlier today. The documentation is strong in this area, so I have added both the answer and the links to the documentation in case community users have similar issues around APIs in the future.
Question - "I'm trying to create a sales order with reference to 'value contract' using SalesOrder API 'SAP_COM_0109'. I am passing over the contract number in payload using field '"ReferenceSDDocument". The expectation is that both the header & line items will be copied over from the contact to sales order. In this case only the header fields are getting filled. Is there a way to automatically copy over header & line items from contract to sales order without adding all item information into the payload?"