YAGS was originally designed as a system for a fantasy setting and was later adapted for use in a Babylon 5 game. Since then it has been used to run horror, more fantasy and a high powered Traveller game. Rather than try to define a single set of rules that covered all of these settings, originally YAGS was forked to provide a specific rules set to cover that particular setting. Later, the rules were merged back together, taking the best changes and discarding the bits that didn't work.
The latest versions of the game continues this tradition, providing a modular framework which can be applied to different settings. Rather than try to define a single character generation system that covers all the possible settings, there is a core set of rules that apply to all settings, and character generation is left as setting dependant. Characters from different settings will be compatible, but not necessarily balanced.
The core rules define what we call Cinematic Realism. They are based in the real world, whilst pushing the bounds of realism much in the way that action films will do so. Whilst YAGS supports playing merely above average people, you can also play the likes of a John McClain or a Beatrice Kiddo. Pushing the system much beyond this level of heroics hasn't been tried.