Categories is a multi-valued field. Any single item can have multiple
categories, so a straight sort is potentially meaningless or at least
doesn't reflect the richness of the data. Instead, you group by Categories.
Companies works the same way.

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Sue Mosher, Outlook MVP
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Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers
http://www.outlookcode.com/jumpstart.aspx
> In task list when I add fields to the view, such as Categories, or
> Companies,
> and I try to sort the list by them, it says "You cannot sort by this
> field".
> Why is that?