Users can use both Planned and Original duration or planned hours. The functions are similar but will change depending on what is happening with the associated task.
Original values represent a task value (Duration or Planned hours) before it was identified as a parent task. 'Planned hours' represents information once that the task has children task or subtasks. In the event a task ever looses all of its subtasks, it would revert to the initial values on the project plan stored as 'Original.'
For example, you have been monitoring Task A's 'Planned Duration' and 'Planned Hours.' Task A has 3 subtasks. During the project execution phase, its decided to move the subtasks to another task on the project, thus making Task A no longer a Parent Task.
Now when you look at Duration and Hour information for Task A, you will look at the 'Original Planned Duration' and 'Original Planned Hours' values that were established before sub-tasks were added and are once again relevant since they are no longer associated with this task.
Original values are the values associated with a task before the work breakdown structure was applied to the project. Once the structure is applied, some tasks will become parent tasks.