User Configuration > Administrative Templates > Microsoft Outlook 2016 > Miscellaneous
User
HKCU\software\policies\microsoft\office\16.0\outlook\preferences
This policy setting allows you to add (append) new categories to the user's current list of categories. If you enable this policy setting, you may add (append) new categories to the user's current list of categories (the default list of categories or the list of categories the user has created). A category's length should not exceed 255 characters. If you disable or do not configure this policy setting, the user's current list of categories is not modified.