This is linked to this article: https://workfront.zendesk.com/hc/en-us/articles/217357397
For tasks, the Duration and the Planned Duration typically show the same value: the length of time between the Planned Start Date and the Planned Completion Date of a task.
When the Duration Type of the task is Effort Driven, the Planned Duration diminishes as you are adding resources to the task.
For example, if a task with a Duration Type of Effort Driven has a Duration of 3 days and you assign one resource with a full-time schedule to the task, the Planned Duration is 3 days, as well.
If you assign three resources with a full-time schedule to the same task, the Duration stays 3 days, but the Planned Duration becomes 1 day. The Planned Duration also changes the Planned Start and Planned Completion dates of the task, to reflect the new Planned Duration. As a result, the timeline of the project is affected as well.
The Effort Driven Duration Type is used for multiple resources assigned to the same task, reducing the time it takes for the work to be completed.
For more information about the Effort Driven Duration Type, see "Effort Driven Duration Type."
Issues and Projects have only a Duration value, which is the difference between the Planned Start Date and Planned Completion Date of the issue, and of the project, respectively.