In order to view the outline levels, follow the steps below:

1. In the View menu, click Gantt Chart.
2. In the Task Name field, select the summary task containing the subtasks you want to show or hide.
3. Go to the Project menu and select the Outline option.
4. To show the subtasks click Show subtasks and to hide them click Hide Subtasks.
5. In order to show all the subtasks, click All subtasks.

You can also show and hide subtasks by clicking the summary task’s outline symbol. Outline symbols indicate whether a summary task has subtasks displayed – or does not have subtasks displayed +.