Apple Numbers '09 - Manual (Page 71)

Chapter 3 Using Tables 71
Creating categories manually
When you create categories by manually inserting a category row, a new column
(the category value column) is added to the table to display unique placeholder
values for each category. The placeholder value is used in the category row to identify
the category.
Category value
column
You may want to hide the column (click its reference tab’s pop-up menu and
choose Hide Column). To change the placeholder name in a category row to a more
meaningful name, edit the name as you would text in any cell.
Double-click to type
a new category name.
Creating categories using values in a column
When you categorize a table using the values in a column, Numbers creates a different
category for each unique value in the column. The column whose value you use to
create categories is the category value column. All rows containing the same value in
the category value column are placed together in a category. The shared value is used
as the category name in the category row.
Category row
Category value column