When you are editing objects in Assess.ai, such as an Item, Test, or Examinee, you'll see a Add Tag field. Click in this field and you will see a drop-down 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 drop-down 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.