Select the message (or messages) to which you want to assign a category.
Open the Edit menu and choose Categories.
The Categories dialog box opens. Click the check box next to any categories that should be assigned to the selected message. (You can choose more than one.)
The Item(s) Belong to These Categories field in the Categories dialog box lists the categories you selected. Click OK to assign the categories to the selected message.
Yet another way to organize messages is to categorize them. Outlook provides several default categories, including Business, Personal, VIP, and others. In addition, you can create your own category. After you've assigned categories to your messages, you can view your messages.
If Outlook doesn't include the category you need, you can add it. To do so, click the Master Category List button in the Categories dialog box. Then, type a name for the new category and click Add.
To view messages by category click View, Arrange By, Category.