Pretty sure there are many answers to that question, with some hitting the mark, and others missing it, but based on what I know, Sixth-Generation is a term that's mostly used for Marketing, and many other groups would contest that definition too, but usually, what would define Sixth-Generation (At least, according to the requirements of various Sixth-Generation Fighter programs) is:
-Advanced Stealth capabilities
-Open Architecture Systems
-Being the centerpiece of programs that would include Collaborative Combat Aircraft
-Are meant to be quarterbacks with said CCAs doing the more frontline work
-Has advanced avionics and sensors that enables it to be part of a Kill-Web or theater-wide integration of data-linked systems that would enable Data-Informed Decision Making (DIDM)
I could be wrong on that, and I could also be missing out on some other features, I would have included "Advanced Engines that are variable in design", but apparently, the F/A-XX is not gonna have those, so it's probably not something that defines Sixth-Generation Fighter development, at least for the F/A-XX.