The Item Module's "Item Groups" tab contains all Items in the database. You can copy Items from other databases, allowing for a standard list of Items to be used across projects within the same organization. When an Item is added to a project, it creates an Occurrence of that Item. Any Occurrence of an Item that is added to a Room will be visible in the Room's Item List.
You can add information on both Item and the Occurrence of the Item. The Information added on the Item level applies to all Occurrence of the Item