Outlook installs with a Master Category List that contains 20 items. You can customize these items by adding or deleting categories. You can also reset the Master Category List to its default list of categories. After you customize the Master Category List, all categories are stored in the Windows Registry. The exact location for Category information in the Registry is HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\Categories.
When assigning a category to an Outlook item, you can choose from any of the 20 default categories or type in your own custom category name in the Categories dialog box (see Figure 7.3).