Magento Open Source, 1.9.x
Root Categories

The category structure is like an upside-down tree, with the root on top. All categories in your catalog are nested below the root. Because the root category is the highest level of the catalog, your store can have only one root category active at a time. You can, however, create additional root categories for alternate catalog structures, different stores, and views. The root category — as a container for the top navigation menu — is not visible to customers in the store, and does not have a URL key.

Root Category in Store Hierarchy