Sometimes you need to define parent-child relations between tasks in the Gantt Chart App. If tasks list is selected as data source, user can select some task in Virto Gantt Chart App and add child task to it. If task has child task it is displayed as a summary task.
If you want to make selected task a child task (sub-task), highlight the parent task which will contain new child task and use “+ Add child task” button. In this case SharePoint list data source must be also a task list.
You can select more than one task and make all of them child-tasks with “+ Add child task” action.