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 based on experimental data or other relationships such as those belonging to the same state or time stamp. An ensemble of models and its representative can either be grouped together or can be separate groups in the ihm_model_group table. 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.