Grouping According to Sort Key 
Each work item carries the two container elements _WI_Object_ID and _WI_Group_ID in its container. Both elements have been defined to hold an object reference.
The container element _WI_Object_ID automatically contains the reference to the object to be processed in the work item
The container element _WI_Group_ID can contain an object reference, which must be assigned to this container element in a binding or via initial value assignment.
The object reference assigned via _WI_Group_ID is generally not identical to _WI_Object_ID, but is derived from the work item execution environment. It is used to group work items that refer to different objects or object types but are nevertheless connected.
Features
Display default attributes in workflow inbox
The default attributes of the objects referenced in _WI_Object_ID and _WI_Group_ID are available in the Business Workplace's workflow inbox under the column headers Object or Group for grouping, sorting, and filtering purposes.
Example
Material master data is processed in several instances of a workflow. The container element _WI_Object_ID in the containers of the individual work items therefore always contains the reference to the object of type BUS1001 (material) to be processed.
The container element _WI_Group_ID is assigned the expression &Material.Laboratory& (laboratory/drawing office for material as object reference) in the relevant steps of the workflow definition. The default attribute of the laboratory is therefore available as a group.
Default methods
The default method of the objects referenced in _WI_Object_ID and _WI_Group_ID is executed by double-clicking in the Object or Group column.