Let the Canonical URL of a category point to an arbitrary URL. This is especially necessary, when a category is very similar to another category (for example by duplication) or you offer the same category in another shop. This prevents your shop from being punished by Google and other search engines due to duplicate content.
Within the category details you will find under "Custom fields" the tab "Category Canonical URL" where you can enter the URL.
Install and activate the plugin as usual in the menu "My Plugins" (Settings -> System -> Plugins).