Are replacement windows the same as new construction?

Replacement windows have no nailing fin or flange and install inside the existing window opening. New construction style windows have this nailing flange and can be installed into existing openings, as long as there is a way to get the flange tight to the existing sheathing. This method of installation is more time-consuming and can require more skill but is often preferred in regards to waterproofing and flashing details that can be offered.