How can I create a category?

From a document:

1. Log in to your account and open the document to which you want to add a new category
2. To the left of the document, click on the plus sign (+) next to Categories
3. Fill in a category name of your choice and click on +or press Enter

From Preferences:

1. Via the menu at upper-right in your account, click on Preferences
2. Open the section categories
3. Fill in a category name of your choice and click on the button Add category.