Hi,
When a document type is created, there is also a DB table (coupled data table) created, where the content of the document type fields is stored. But to the document type belongs also some instance of a document (node) object.
In this
article you can see the structure of the documents in database, so you need to update the records in CMS_Document and CMS_Tree tables, if you want to use it with documents. I hope it will help.
Best Regards,
Juraj Ondrus