The User tab offers you one place where you can create custom fields. The five field names (User 1-4, Categories, and User Status) come from Microsoft Outlook and will sync with Outlook.
The field names for User 1-4 can be customized and use drop-down lists for all values. Customize them by double-clicking on them on this tab. Or you can re-type them on the User Interface tab in Tools | Options.
Read More > User Interface Tab
You can also use Saved Groups for greater flexibility to help you manage your contacts. You can have an unlimited number of Saved Groups, and any contact can be in any group (and as many groups as you want). You can do Lookups on Saved Group members. For these reasons, using Saved Groups may be more helpful than using the User fields.
Read More > Groups / Saved Groups