A Mapped Data Object is simply a series of bytes. A subset sequence of these bytes can compose a logical structure. Some structures can be explicitly identified by the presence of enclosing lengths and some form of identifier that prefaces the data. Other logical structures might have no explicit identification but are defined only by external knowledge of the meaning of some sequence of bytes. Whatever the case, the physical representation of the data must be known to Mapping Services so that it can translate any NCL reference of a logical structure into the correct access technique corresponding to the physical data construction.
The system provides default rules for structuring data. If an MDO is created and always processed by NCL, you can choose to accept the default physical structure provided by the system. However, if an external data source is to be processed, or if a specific format needs to be devised for communication with some other program, then control of the physical representation is necessary.