A unique identifier for a collection or group of structural models. 
      This data item can be used to group models into structural clusters
      or using other criteria. 
      A cluster of models and its representative can either be grouped together
      or can be separate groups. The choice between the two options should be 
      decided based on how the modeling was carried out and how the representative 
      was chosen. If the representative is a member of the ensemble 
      (i.e., best scoring model), then it is recommended that the
      representative and the ensemble belong to the same model group. If the
      representative is calculated from the ensemble (i.e., centroid), then it is
      recommended that the representative be separated into a different group.