I know that the datatype docs are lacking, so I though I could at least help you out by explaining the related object dataype.
When creating an object of a class which includes and attribute of the related object datatype, one is given the option to browse the hierarki of existing objects and select a related object. This can be useful in many cases, for instance when relating an author object to an article object.
As for the subtree subscription datatype, I'm not too familiar with it, but I think its purpose is to allow the creator of an object to check it and then be notified of objects which are created below it in the hierarki.
