Apple Numbers '09 - Manual (Page 74)

To create a subcategory, open the Reorganize window, and click the Add (+) button m
adjacent to the category or subcategory below which you want the new subcategory.
Then choose the column whose values you want to use for the subcategory.
Click to add subcategory.
Each subcategory has its own category row in the table.
Subcategory row
To add a category or subcategory above or below an existing category or subcategory, m
in the cell reference pop-up menu for the category or subcategory, choose Add
Category Above or Add Category Below.
If you delete a value from a category value column, its row is placed into a category
with blank values in the column.
74 Chapter 3 Using Tables