From the result of you filter selection, you click on the Group aggregation button, this will add together all parts matching the current filter selection and show it as one history and one forecast, in the Forecast workbench in the Seasonal Profile workbench the seasonal profile will be computed based on the total history as well. The access level to the forecast of the group sum will be dependent on the access level of the individual items. All items will have to have Full Access for the user to have Full Access privileges of the group selection, if one item has access level Evaluator then the group selection will also have this access level. Then the part with Evaluator access will have to be in status released or reviewed for the group selection to be visible.
The sum of the history and forecast of the filtered items will be shown in the Selected Workbench (Forecast or Seasonal Profile)