A summary task is a main task comprising subtasks. It is often used to identify the major and minor phases of your project. For instance, the summary task "Lay the foundation" could include the subtasks "Dig a hole" and "Pour the concrete."
Like a milestone, a summary task requires no work; it’s not a detailed set of actions that someone actually performs. But unlike a milestone, a summary task summarizes information about its subtasks. For example, if you assign costs to your tasks, a summary task’s cost is the sum of its subtasks’ costs.
Regardless of how many tasks are in your project, you can show a very detailed hierarchical structure among tasks. Microsoft Project enables you to create summary tasks containing subtasks that are themselves summary tasks.
The advantage of creating an outline of summary tasks and subtasks is that it allows you to view and print your project plan at an appropriate level of detail. You can hide and show subtasks as you see fit. For example, if you hide your subtasks so that only the highest-level summary tasks are shown, you get a broad overview of your project. Then, you can show just the summary tasks for the current phase of your project and print that view of your plan for your next meeting.