Summary Tasks are specific to grouping rows (Milestones, Tasks and Summary Tasks) in Gantt chart page for better user visual experience.
Summary Tasks rows can be expanded/collapsed, either all or at individual level. Expand/collapse is not available if rows are filtered or not in default standard summary task order.
Selected set of contiguous rows can be moved underneath summary task to create grouping which will be presented with indent and light grey separator line. Rows can be moved out from the summary tasks if needed by using outdent.
