Tasks can either be manually or automatically scheduled.
Automatically scheduled tasks' start and end dates are automatically updated based on the task constraints, dependencies (predecessors or successors), and position in the plan hierarchy. This means that a task start and end date can be updated as soon as the task is added to a plan.
Manually scheduled tasks are manually adjusted by the user.
By default, tasks are automatically scheduled. To manually schedule a task, go to the task contextual menu > Advanced Edit > Advanced and check the Manually Scheduled check box.
You can also set automatic or manual scheduling at the plan level. Add the Default Task Mode column to the view (right-click then select Default Task Mode). Then select Auto Schedule or Manual Schedule.
To exclude a task from scheduling, Go to the task contextual menu > Advanced Edit > Advanced and check the Inactive check box. Inactive tasks do not push their linked tasks or rollup their attributes to parent events.