The following paragraphs defines the concepts of object reference, object reference chain, and topmost object reference. These concepts are of high importance within the XML request/response system.

An object reference is an identification of an individual XML request defined by a unique triple of data, namely

• timestamp (UNIX time) defined at the moment of generation
• date of the object defined by the provider
• id of the object defined by the provider

Each and every XML request of version 4.5 is defined by such an object reference. All XML responses are replies to the object reference of the corresponding request.

Thus the object reference is a clustered primary key of the request/response system!

As a consequence of the XML schema coded business logic most XML request schemas define more than just 1 object reference, e.g. a "HospitalMCDRequest" needs 2 object references, namely

• a mandatory invoice object reference from the "General Invoice Request" process
• a mandatory mcd object reference from the " HospitalMCDRequest" process

This set of object references by itself defines an ordered object reference chain (e.g. invoice -> mcd) with the topmost object reference as unique identification of the XML request. Therefore the object reference chain defines -on business logic point of view- dependencies among different XML requests.

The end node of an object reference chain is defined as topmost object reference, that is by definition the object reference of the XML request itself. Each request/response communication is always done for this topmost object reference and only for that.