Steve,
It looks something like this in the EDD:
Element (Marker): IndexTerm
Initial marker type
In all contexts.
Use marker type: Index
You can find the details in the structure applications guide.
In a structured document, the user does not insert markers. As with other elements, the user inserts the element itself. FrameMaker opens the marker dialog in which the user enters the desired content. The user CAN change the marker type but should not, especially if the EDD has several different marker elements.
I do not know what life would be like if your EDD had ONE marker element and then allow the user to specify the marker type. That may lead to confusion and is contrary to the concepts of structured writing. Maybe others can offer their views.
Van