Purpose: Some tasks have a client or environment-imposed date constraints. You should identify these date constraints and insert them in MSP, so that it can take care of date calculations.
You may want to have control over tasks dates for some of the tasks. You can manually schedule these tasks and make them independent of any other tasks. MSP puts a constraint on manually scheduled tasks as soon as you associate a date with them. You must make a note of these tasks and their constraints, as MSP will not change them automatically.