NOW: Usage Page

The New Overlay Wizard (NOW) opens and displays these pages, in order: Properties, Usage (this page), Impact, and Impacted Activities.

Select the appropriate settings on this page, remembering that some selections may deselect others.
Click Next.

The settings on this page are:

While Building A Forecast use This Overlay In The Following Way: radio button group

Calculate Using The Specified Initial and Ending Impact—Use the specified initial and ending impact when building a forecast. This options is the default.

This option is missing from the display, and thus disabled, if you selected This Overlay Shall: / Redistribute Forecast Without Changing Total (on the Properties page).
Always Use Entered Distribution—Use the specified (entered) distribution when building a forecast.
Always Calculate Disregarding Impacts and Distribution—Use this overlay, but always disregard impacts and distribution when building a forecast.