Demand and scheduled resource allocations
When assigning resources, you can assign either demand or scheduled resource allocations. A demand allocation is an estimate of how much of a particular resource or skill is required to complete a task; you might assign demand allocations during your initial project planning phase. A scheduled allocation is an allocation of a resource that will actually work on the task; you might satisfy demand allocations with scheduled allocations once you know the resources available to your project.
By default, resource folders are assigned as demand allocations and individual resources are assigned as scheduled allocations. This is because a resource folder often represents a skill or type of resource, whereas individual resources are often people or specific resources. For example, you might have a resource folder called Engineers containing a list of all the engineers, including Nigel Foster. When you assign Engineers to a task, that allocation shows that there is a demand for Engineers to do work on that task. When you assign Nigel Foster to the same task, his scheduled allocation satisfies the Engineers demand allocation.
You can specify whether demand allocations should be deleted once they have been completely satisfied or whether they should remain in the project, using the Remove demand allocations when fully satisfied check box on the Assign tab of the Options dialog. Select this check box if you want to delete demand allocations once they have been completely satisfied, or clear the check box if you want them to remain.
Usually when you assign a resource to a task, the resource's default allocation is assigned. However, in some circumstances when you assign a resource, its default allocation is overridden. For example, if there is a demand allocation for 25 man hours of Engineers, and you satisfy this demand with Nigel Foster whose default allocation is 35 hours, only 25 hours of Nigel Foster are assigned to satisfy this demand.