When should I select the billable percent reason?

When you create a resource request for a billable assignment in the Manage Project Resource Requests or the Edit Project Resource Request pages, and if you have assigned the billable percent for this planned resource as less than 100%, you can also select a reason for the percentage specified. Suppose you have a junior resource who's still learning, you might want to assign only 40% of the resource's time to the request and select the reason that the resource is a shadow resource.