I think it’s whether or not it attempts to stay within physics. It can still be very out there, like fusion torch rockets or mind uploading, but there is nothin

I think it’s whether or not it attempts to stay within physics. It can still be very out there, like fusion torch rockets or mind uploading, but there is nothing happening that is physically impossible according to what we know about the universe. A physicist would not have to suspend disbelief (much). This is also called hard vs soft sci-fi where very soft is fantasy. Fantasy incorporates elements that go beyond any known physics. Sometimes they are explained in world with invented physics like Star Trek, and sometimes they are alien and not explained like the stuff the protomolecule can do in The Expanse. If it’s alien it’s playing with “what if we met someone WAY more advanced than us?” It’s not a binary thing. Some stories are super far out in fantasy like Star Wars while others try to be very scientifically accurate like For All Mankind, Gattaca, or most of the human tech in The Expanse.