A document type group (or simply a document group) is a classification of document types according to some common characteristic or subject. You could, for example, decide to group different kinds of financial reports into a single document group. Each document group, then, contains one or more document types. A document type is defined by its layout or structure. An invoice, for example, is a document type.
Documents are classified in document groups as a means of organizing information hierarchically and thus speeding up the search process. In other words, when users perform a search, they typically select a document group first, and then select a document type or types from that group to search.
In addition, all document types are assigned to storage groups. The storage group you select during configuration is a default storage group. You can assign document types in the same document group to different storage groups. You would take this approach, for example, if you do not want to treat all the document types in the same way.