Behavior allows you to change how certain tasks are represented in the chart.
Treat parent work items as groups
A group aggregates values from the tasks it contains, and its own values for manual start date, finish date or work estimation are ignored. You can configure your Gantt chart to show parent work items either as actual tasks or as groups (containers for tasks).
Gantt groups work items by default. To have your parent work items shown as actual tasks, on the Scheduling screen, under Groups, uncheck the "Treat parent work items as groups" checkbox.
If a work item is treated as a group, any resources assigned to that work item will be ignored when calculating resource usage.