Nail fin frames are considered new construction or original type frames. They are nailed to the framing members of your home and with replacing this type of frame it requires that the old frames be removed from the existing opening by cutting back the original exterior material (stucco or siding) to remove the old frames, This method can have a higher installation cost factor based on the structures exterior
Retrofit type frames are designed to fit within the existing window frame (aluminum or wood). This frame type does not disturb anything on the exterior or interior, leaving the existing weatherproofing around the original window intact. You will loose some glass area using the retrofit type window frame. The retrofit window method has a lower installation cost factor than the nail fin window method..
No matter which frame type you choose to replace your existing windows, measurements should be taken by the contractor who is installing your new windows. Measurements are very critical to the proper installation of your new windows and doors.