In Project, an indented task becomes a subtask of the task above it, which becomes a summary task. A summary task is made up of subtasks, and it shows their combined information.
To create a subtask or a summary task, indent a task below another one. In the Gantt Chart view, select the task you want to turn into a subtask, then select Task > Indent.
The task you selected is now a subtask, and the task above it, that isn’t indented, is now a summary task.
: Select Outdent 
Show or hide subtasks
To show or hide all subtasks for all summary tasks in Project, in the View tab, select Outline in the Data section, and then select All Subtasks to show all the subtasks. You can also select one of the Level options below it to show all the subtasks up to that level.
To show and hide all subtasks for a single summary task, simple select the expand or collapse button to the left of the summary task name to show them or hide them respectively.
: Outline numbers can be useful for viewing the organization of your tasks at a glance. Learn how to add or hide outline numbers.
Tips for organizing and working with outlined tasks
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If your tasks aren’t indenting or outdenting, there could be a variety of reasons why. Learn more about indenting or outdenting your tasks.
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When organizing the tasks for a project, you should plan the outline for the project in one of two ways; the top-down method or the bottom-up method.
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With the top-down method, you identify the major phases first and then break the phases down into individual tasks. The top-down method gives you a vision of the plan as soon as you decide on the major phases.
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With the bottom-up method, you list all the possible tasks first, and then you group them into phases.
: After you finish organizing them, don't forget to link your tasks. Subtasks and summary tasks create structure, but they don’t create task dependencies.
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You can include the project as a summary task. Learn how to show a project as a summary task in the task list.
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When you move or delete a summary task, Project moves or deletes all of its subtasks. Before you delete a summary task, outdent the subtasks you want to keep.
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You can change the duration of a summary task without changing each subtask. But be careful — changing the duration of the summary task does not necessarily change the durations of the subtasks. Learn how to change the duration of a summary task.
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Avoid assigning resources to summary tasks. Assign them to the subtasks instead, or you might not be able to resolve overallocations.
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Summary tasks don’t always add up. Some summary task values (cost and work) are the total of the subtask values, others (duration and baseline) aren’t. Learn about how Project schedules tasks.
