MstnPropertyFormatter:AreaClass has nothing to do with closed elements. That schema is all about formatting property values.
Closed elements are represented by subclasses of BaseElementSchema:MstnClosedBoundary.
Create an ECInstance of AreaClass, set its Units property to -3, and pass it to TextField (or to IDgnECTypeAdapter, if you're doing your own formatting).
Closed elements are represented by subclasses of BaseElementSchema:MstnClosedBoundary.
Create an ECInstance of AreaClass, set its Units property to -3, and pass it to TextField (or to IDgnECTypeAdapter, if you're doing your own formatting).