A group is a set of entities all having the same value in a field. This field is called the "group by field". For example, the set of Bug entities having the value "Low" in the BG_SEVERITY field is a group.
A group may also contain subgroups. For example, a group of Bugs grouped by BG_SEVERITY may have subgroups grouped by BG_RESPONSIBLE.
  FieldName | The name of the field by which the objects are grouped. | 
  FieldValue | Items belonging to the group have this value in the field. | 
  GroupList | The list of sub-groups of this group. | 
  ItemCount | The number of objects in the group. | 
  ItemList | The list of references to the members of the group. | 
  TotalCount | Total result count, when fetch limit is used in filter. |