Reusing type instances for documents.
Some document types categorize their document instances by types. That is the document type Topic has a type property that refers to a Topic Type instance. So if you write a new article on some topic, you could use the Topic template and set the topic type property to Article.
To give you an easier start projectdoc Prototypes (en) provides examples for these types you could use in your project. For an example what we use for our spaces, have a look at the smartics index space.