Apple Numbers '09 - Manual (Page 109)

Chapter 4 Working with Table Cells 109
When you type this value into a cell with the
above format The value is displayed like this
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To create a custom number format that has conditions:
1 Select one or more cells.
2 Do one of the following:
Choose Custom from the Cell Formats pop-up menu in the format bar. Â
Choose Custom from the Cell Format pop-up menu in the Cells inspector. Â
Choose Format > Create Custom Cell Format. Â
3 From the Type pop-up menu, choose Number & Text.
4 Click the Add (+) button to the right of the format field to add a condition.
5 From the pop-up menu above the new format field, choose a condition.
6 Define the number format you want to apply when a number meets the condition by
adding elements to the format field.
See “Creating a Custom Number Format” on page 102 for information about
interacting with the format field.
7 Repeat steps 4 through 6 as needed to define all your conditions.
Note: To remove a condition, click the Delete (–) button to the right of its format field.