There is a way you could pull this off. Create a "classification" attribute. Link it as a parent to another attribute, something like Document Type. Add the classification name to the Matter Type column, and default Classification from Matter Type on the attribute definition dialog. This way, when users upload new documents, when they choose matter 1234, it will default to the classification LIT, which then allows them to choose Document Types belonging to LIT, or if it defaulted to PROP, they would be able to choose Document Types related to PROP. I think this is what you are looking for. However, it would only really apply to new documents. When users re-profile existing documents, for example, the classification won't be defaulted based on matter.