From a document:
- Log in to your account and open the document to which you want to add a new category
- To the left of the document, click on the plus sign (+) next to Categories
- Fill in a category name of your choice and click on +or press Enter
- Via the menu at upper-right in your account, click on Preferences
- Open the section categories
- Fill in a category name of your choice and click on the button Add category.