A Building Block stores information about any Zones, Data Sources, E-mail Settings and linked User Settings associated with an object. At import, these links are also established in the target site. The associated objects are created in the target site if they do not exist there.
If you create a Building Block that includes an object containing resources (such as desktop pictures), an additional (XBB) file is automatically created that contains these resources. When you import this Building Block into a target site, the items in the XBB file are also added to the target site.