Customize which TFS field is used as description

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This is already possible. You need to create your own configuration file for your project. You'll find more information about creating such a file on this blog post.

The description field is configured at the top of the document. You can configure a description field for each visible work item type.

I already created a custom configuration, based on MS Agile, but can't find examples of how to set the description field for each work item type.
Can you provide an example?

To specify the field to use as the description field, you should add a "DescriptionField" attribute to the "WorkItemType" element, under the "VisibleWorkItemTypes" section at the beginning of the mapping file.

The default MSF Agile 5.0 actually defines the Microsoft.VSTS.TCM.ReproSteps field as the description field for the Bug work item type.

If no field is specified, then Urban Turtle defaults to System.Description.