I'm a co-inventor of STAR voting. I explain it as, STAR voting is score voting on a 0-5 scale followed by an automatic ("instant") top two runoff. Whereas approval voting is score voting on a 0-1 binary scale. STAR voting performs a tiny bit better in computer simulations of voter satisfaction, but approval voting has the practical benefit of being super simple and requiring no voting machine upgrades. Both are superior to IRV.

Here's the first email I received from Mark Frohnmayer where he discussed STAR voting, following my speaking at his conference at University of Oregon, that gave birth to it. :) https://twitter.com/ClayShentrup/status/1278118075972202496