Bill to code setup through DI API

Hi,

When trying to to create a sales order through Documents object in DI API,

I can pass in a ShipToCode to choose different ship to address that has been created in Business Partner. I want to implement the same function like choosing different bill to code, but I can't find a Bill to code to setup in this Documents object.

I know that I can use AddressExtension to pass in both bill to and ship to address.

Any information plz suggest me...

Thanks