Tagging Media is useful when there is a piece of content that should be in multiple Groups & Categories.
When you have a file that needs to be added to another category, click on the edit button.
Click on the "+" in the Categories and Groups section.
A window will open up and you will see where the piece of content currently is tagged.
If a user would like to keep the piece of content in the same group that it is in, simply click in the small box to tag it to another category. If a user would like to tag it to a new group and category, click on the group in the left side box and the categories will populate in the right side. Click inside the box to check it into another category. Select as many categories as you wish. Click "Add" when finished tagging to the categories. Users will then be returned to the Media File screen.
As always, press save when you are finished!