An identifier to group structural models into collections or sets. A cluster of models and its representative can either be grouped together or can be separate groups in the ma_model_list 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. If the models do not need to be grouped into collections, then the _ma_model_list.model_group_id is the same as _ma_model_list.model_id.