There's a ton of info here: https://www.schrothr....com/technology on materials, stretching, homologation, etc., from one company's point of view, of course.
I just learned from that page: "The biggest change required in SFI 16.6 versus SFI 16.5 is the addition of a mandatory Negative G belt, making all belts either 7 or 9 point configurations. The Negative G belt is a center mounted sub-strap that acts to hold the camlock and lap belts in position on the pelvis bones during a roll over or negative G incident."