When you are editing objects in Ada, such as an Item, Test, or Examinee, you'll see a Add Tag field. Click in this field and you will see a dropdown of available tags. If you have 2 or more levels deep, they will be separated by a period (dot); this was selected because it is commonly used for educational standards such as the Common Core State Standards in the USA. If you start typing, the dropdown will autocomplete, and automatically narrow the possibilities.
You can add multiple tags because they are orthogonal. Here, a second tag has been added for Ethnicity.