Nina,
In an EDD, every element has a "type," because certain elements represent special FrameMaker elements such as marker objects. Along with this, during import, the read/write rules alert FrameMaker what type of element each one is in the XML. What you did is told FrameMaker that the shortdesc element should be a marker-type element, but the EDD still defines shortdesc as a container-type element. Declarations in the read/write rules must match the declarations in the corresponding EDD.
Again, as I mentioned in another thread, this is a very broad topic. I think that you should spend some time with the Structure Developer's Guide, which contains all the information about these subjects. We are happy to assist with focused questions that can be clearly answered through brief forum exchanges, but the answers to your questions require a fundamental elevation in subject matter expertise that we can't provide here.
Russ