The one needed for the polygon branch is the preview entity,
which is used to hold tessellation information for visual feedback
during a geometric operation.

+ Preview entities may be related to model and/or mesh entities;
+ they may have text notes attached to them to be shown on mouse-hover
  or in a list that is tied to rendering (so that brushing the list
  highlights the preview geometry and vice-versa);
+ they may be associated with attributes, but there is not a use case for this yet;
+ they may have properties associated with them indicating that one
  of several preview entities should be selected to disambiguate an
  operation.