Which file determines what elements are valid in a ditamap

Thanks Gareth. The reason I ask is that appendix was removed from ditamap as a vailid element. In the default style sheet there is still an element that is labeled appendix. If I turn off context rules it is not even in the list of options. So my thought was to add the appendix element back to ditamap. I will look at dita specialization, I expect this is something that would be easily handled by it.

Bryon