Appendix B -- Advanced DXF Issues
When filing a stream of group data, a single object may be composed of several filer members, one for each level of inheritance where filing is done. Since derived classes and levels of inheritance can evolve separately, the data of each class filer member must be segregated from other members. This is achieved using subclass markers.
All class filer members are expected to precede their class-specific portion of instance data with a "subclass" marker-a 100 group code followed by a string with the actual name of the class. This does not affect the state needed to define the object's state, but it provides a means for the DXF file parsers to direct the group codes to the corresponding application software.
For example, an object that has data from different derived classes would be represented as follows:999