When going with a retrofit installation, you need to consider what kind of frame is appropriate for your install. If you're going to install straight onto stucco siding, then a dual wall retrofit fin is appropriate for that application because then you actually have a surface to properly caulk against.
Now, if you have one by a trim around your windows, you're gonna need a single wall fin, and that way it doesn't protrude past your trim. Weakening that caulk line and leaving you with an ugly installation. Now, this is something that our design consultants can go over with you and make sure that you're making the right choice for your install.