You can supplement Microsoft Outlook's built-in Categories field with your own Subcategory field, or use a category naming convention to allow you to better organize and group "subcategories" together.

Method #1 | Method #2 | Method #3 | More Information

Method 1 - Custom Keyword field

Add a new Subcategory field to a folder like this:

1. Open a Contact.
2. Click the All fields tab
3. Select User-defined fields in folder.
4. Click New.
5. Define your new field as follows:
   Name: Subcategory
   Type: Keywords
   Format: Text

The Keywords type allows you to apply multiple subcategories to each item.

Limitations of this method:
You cannot group by more than one Keywords-type field in a view. This means that you cannot have a view that groups by Category then by Subcategory. You can create a filtered view for the Category then group by the subcategory.

Because you can't create a custom email form for incoming items, if you need to apply subcategories names to email items, you can either create a custom keyword field in the Inbox (use in-cell editing to add keywords) or use two category names and filter so you only see items assigned to the main category name, then group by category to group by "subcategory". Or use the naming convention method below.

Method 2 - Naming Convention

Another method is to name your categories so they include subcategory information -- for example, Business - Local, Business - Regional, Business - International, etc. When you use this method, you can sort by category name and group by category, so that the categories will be grouped alphabetically, with all of the Business categories together (from our example).

Method 3 - Colors

Use Outlook's Color Categories feature to "group" categories together. Each color is the "master category" and the category names are the subcategories. This method is less useful because you can't group or filter by colors.