Magento Open Source 2.2.x
Root Categories

The products in the main menu are determined by the root categoryA set of products that share particular characteristics or attributes. that is assigned to the store. The root category is basically a container for the main menu in the category tree. You can create a root category with an entirely new set of products, or copy products from an existing root category. The root category can be assigned to the current store, or to any other store in the same website.

From the AdminThe password-protected back office of your store where orders, catalog, content, and configurations are managed., the category structure is like an upside-down tree, with the root on top. The root has a name, but no URLUniform Resource Locator: The unique address of a page on the internet. key, and does not appear in the top navigation of the store. All other categories in the menu 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 following example shows how to create a new root category and assign it to a different store.