Constraint

A constraint is a requirement that is set for a task's start or finish date. A constraint affects how a task's start and finish dates will be affected by other tasks in a task plan, based on the relationships between the tasks and the schedule for the task plan itself. There are two types of constraints: flexible and inflexible. A flexible constraint is one that does not require a specific start or finish date. An inflexible constraint is one that does require a specific start or finish date.