1 Science in Genre Only: The work is unambiguously set in the literary genre of Science Fiction, but scientific it is not. Applied Phlebotinum is the rule of the day, often of the Nonsensoleum kind, Green Rocks gain New Powers as the Plot Demands, and both Bellisario’s Maxim and the MST3K Mantra apply. Works like Futurama, Star Wars, Tengen Toppa Gurren Lagann, The DC and Marvel universes,note Doctor Who, and The Hitchhiker’s Guide to the Galaxy fall in this class.
2 World of Phlebotinum: The universe is full of Applied Phlebotinum with more to be found behind every star, but the Phlebotinum is dealt with in a fairly consistent fashion despite its lack of correspondence with reality, and in-universe, it’s considered to lie within the realm of scientific inquiry. Works like Neon Genesis Evangelion, the various Star Trek series, and StarCraft fall in this category.
As for The Expanse, the linked page rates it as a 4:
One Big Lie: The author invents one (or, at most, a very few) counterfactual physical laws and writes a story that explores the implications of these principles. Consider, for instance, Cities in Flight’s “Dirac Equations” and “spindizzy motor” leading to instantaneous communication, or Mass Effect’s “Element Zero” being the basis for all of the series’ futuristic technology. Other works in this class include The Expanse series, the Ad Astra board games, Robert A. Heinlein’s Farnham’s Freehold, and many of Vernor Vinge’s books.
Compared to Star Wars, it’s harder on the Sci-Fi hardness scale up to 6.
I wonder where The Expanse or Sprawl would fall…
As for The Expanse, the linked page rates it as a 4:
Gotta love TVtropes.