In most situations, you can schedule tasks to occur at the right time by listing them in logical order, linking them, and adding lead or lag time. Performing these three actions gives your schedule maximum flexibility. If a task starts earlier or later than planned, all the succeeding linked tasks begin earlier or later, too.
Occasionally, a task must start or finish in relation to a specific date to successfully complete the project. For example, the planner for the clockwork exhibit knows that the museum work crew won’t be available to paint the gallery walls until January 28, so the planner specifies that painting start no earlier than January 28. By doing this, the planner has changed the constraint, tied to a specific date, to the painting task.
Every task in Microsoft Project has a constraint applied to it. If you don’t apply a constraint, a default constraint is applied for you. For example, the As Soon As Possible constraint, which is the default constraint, tells Microsoft Project to schedule a task to start on the earliest date possible, taking predecessor links, lag or lead time, and other scheduling factors into account. Normally, you let Microsoft Project calculate the start and finish dates for a linked task, leaving it to recalculate these dates as the schedule changes.