A task can be either Manually Scheduled or Automatically Scheduled. Manually Scheduled tasks have user-defined Start, Finish ...

A task can be either Manually Scheduled or Automatically Scheduled.

Manually Scheduled tasks have user-defined Start, Finish and Duration values. Project will never change their dates, but may warn you if there are potential issues with the entered values.

Automatically Scheduled tasks have Start, Finish and Duration values calculated by Project based on dependencies, constraints, calendars, and other factors.