Greenstone 3 digital library has concept of collection. Collections can contain various documents like books, articles, image collections etc.
All collections present at the main page of digital library site. This page could look messy with a lot of collections.
To structure collections I added collection groups service. One may control collections structure, add and describe groups by editing file http://trac.greenstone.org/browser/main/trunk/greenstone3/web/sites/localsite/groupConfig.xml
Group hierarchy is described inside <hierarchy> xml element.
Group described inside <groupDescriptions> xml element.
Group could have name, description, short description and path to group image. If you break groupConfig.xml xml syntax by mistake it will be ignored.