Categories enable you to manage the items in your mailbox. To use categories, you must be using the Preview Pane. To add a category to an item:
In the Preview Pane menu bar at the top, click .
Select the categories you want to apply to the item. The category color appears next to the item in the items list.
If you do not see a category you would like, click Manage at the top. You can then create and delete categories.