Milestone in Task Decomposition refers to a scheduled event or stage that indicates completion status of sub-tasks and their goals which belong to one and the same task breakdown. Milestones create the second of the four possible levels in task decomposition. Milestones reflect tangible and observable results received upon completion of related sub-tasks and goals.
Milestones cannot be measured or estimated because they never consume resources and never have timeframes. They just reflect completion status of related sub-tasks and goals. They are often used in task timelines and schedules for status tracking purposes.
Meanwhile, milestones in task decomposition can be used to measure success of sub-tasks or goals at any given point in time. A milestone can be linked to a certain task, and when this sub-task is done the milestone is regarded as reached. This milestone may include some criteria that determine success of the task. Examples of success criteria are: performance vs. quality, results delivered vs. resulted expected, time elapsed vs. time planned.