1. A computer readable medium including program code for providing a message-based interface for a document output request, including a set of output requests related to a business document, the medium comprising: program code for receiving via a message-based interface derived from a common business object model, where the common business object model includes business objects having relationships that enable derivation of message-based interfaces and message packages, the message-based interface exposing at least one service as defined in a service registry and from a heterogeneous application executing in an environment of computer systems providing message-based services, a first message for requesting output of a business document, including a set of output requests related to a referenced business object, the output requests using one or more output channels for sending out documents, the first message including a message package hierarchically organized in memory based on and derived from the common business object model, the first message package including: at a first hierarchical level in the first message package, a document output request message entity; at the first hierarchical level in the first message package, a message header package including, at a second hierarchical level in the first message package, a message header entity; and at the first hierarchical level in the first message package, a document output request package including, at a second hierarchical level in the first message package, a document output request entity, wherein the document output request entity includes, at a third hierarchical level in the first message package, an access control list entity, a universally unique identifier (UUID), a reference object node identifier (ID), a reference object type code, and a reference object node type code; program code for processing the first message based on the hierarchical organization of the first message package, where processing the first message includes unpacking the first message package based on the hierarchical organization of the first message package, the specific grouping and order of the hierarchical elements, and the first message package's derivation from the common business object model, wherein the particular hierarchical organization of the first message package and the specific grouping and order of the hierarchical elements are used at least in part to identify the purpose of the first message; and program code for sending a second message to the heterogeneous application responsive to the first message, where the second message includes a second message package derived from the common business object model to provide consistent semantics with the first message package.