You have the ability to add classification fields in the backend of Sitefinity in the User profile section. In order to do this you will have to go to Administration > Users > Manage Profile Types and then select the "Basic" type. Under there you will have the UI for specifying custom user profile fields. Classification will be available there. Once you have a custom fields defined you can check out this blog post
. The post is specifying how to bind user profile taxons to a RadAutoCompleteBox but you can see there how you would grab the taxon field from the user profile.
All the best,
the Telerik team
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