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When you update the Fixed Duration value for a task, it is given a Fixed Duration, independent of the work required to complete the task.
This task can be automatically scheduled, or you can manually assign a Start or Finish date.
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With fixed duration, the amount of work that needs to be completed for a task (it's its work estimate) is divided evenly across the the duration when calculating resource usage. So if an issue's work estimate is 10 hours (1d 2h) and the task has a fixed duration of 10 working days, the issue would require 1 hour of work each day.
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