Tags are the ability to assign an identifier to an object. Tags can be assigned to contacts, tasks, custom data, documents, and file notes. You can create, edit, and delete the tags in your database by going into Admin > Additional Settings > Tags.
How to add a tag to documents is very simple. Just open any matter that has documents. Once in the documents folder find any document and select it (dont open). Once selected you have two options either right-click and select propertise or once selected, select properties under the matter icons.
Once you open propertise at the very bottom there are your tag options. Select the once you want and select save.