1. A computer readable medium including program code for providing a message-based interface for exchanging information about instances of a user-specific categorized marker with an assignment to a business object instance, the medium comprising: program code for receiving via a 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 querying information about flags that represent instances of a user-specific categorized marker with assignments to business object instances, the first message including a message package hierarchically organized as: a flag query message entity; and a flag package comprising a flag entity, where the flag entity includes a universally unique identifier, an assigned business object node reference, a category code, a usage context reference, system administrative data and a key, and where the flag entity comprises a creation identity entity; and program code for sending a second message to the heterogeneous application responsive to the first message.