Activating Supplementary Data View Content Item as Assingment Attribute

Hello All

I am trying to create a new Assignment attribute using a content item from a Supplementary data source.

A new data source has been created

A new view has been created and registered against the new data source at the Display Area = "Header and Footer"

The data source is disabled.

When I navigate to Configuration > Data Sources > Select the new supplementary data view > This takes me to a form where I can select whether the content item is a "Template Item" BUT I can NOT see an option to select the template items as "Assigment Attributes".

(see attached screen shot)