Link: Difference between - Nail On / Retrofit 

What’s the difference between a nail-on window and a retrofit window?

A retrofit window is a window that is custom built to fit within the opening of an existing window. It is built to fit precisely into the opening and can be installed without disturbing the interior and exterior areas around the window.

A nail-on window is a window that is made to be nailed onto the frame of a house. In order to do this, the stucco around the outside of the window must be broken. The nail-on window is then nailed onto the existing wood members. New waterproofing paper is applied along with new lath and stucco.