I noticed that lately there are some Sketch UI Kits available on sites like Dribble to design iOS screens. These, when downloaded and opened complain that I am missing a font called "SFUI" (in several weights). I have El Capitan, and I know the included SF fonts are for UI usage and don't show up in font menus,
So what's this font they are using in their Sketch files?
You can download SFUI here: https://developer.apple.com/fonts/
The one marked "For iOS 9 and OS X El Capitan" is SFUI. (watchOS uses SFCompact)