1. A tangible computer readable medium including program code for providing a message-based interface for performing a budget availability control register service, the budget availability control register service allowing a user to determine available and consumed budgets on an assignment, 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 from budget processing for querying a budget availability control register for available and already consumed budget values for an account assignment with possible restrictions on a time period and budget relevant attributes that includes a first message package derived from the common business object model and hierarchically organized in memory as: a budget availability control register enterprise resource planning item by elements query message entity; and a selection package comprising a budget availability control register item selection by elements entity, where the budget availability control register item selection by elements entity includes a fiscal year ID and at least one selection by budget availability control register code, where each selection by budget availability control register code includes an inclusion/exclusion code, an interval boundary type code, and a lower boundary budget availability control register code; program code for processing the first message according to the hierarchical organization of the first message package, where processing the first message includes unpacking the first message package based on the common business object model; 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.