What does SF stand for? Perhaps your first thought is Science Fiction, but these days it actually means Speculative Fiction with several subcategories.

Science Fiction is the heavy hitter, of course, which can be broken down into several other subsubcategories:

• Hard Science Fiction – This drives deeply into technology and typically engineering science, striving to get it right to a technical degree. The Martian is an example. There’s nary a deflector dish in there.
• Character Science Fiction or Soft Science Fiction – Here, the science is pure fiction and the emphasis is on the people and themes. Star Trek, Frankenstein, Dune (though Dune gets into some deep terra-forming, it doesn’t try for accuracy). Soft Science Fiction can also refer to the “soft” sciences, like psychology, sociology, anthropology or archeology.
• Time Travel – This can fall into either of the above category, but there’s so much of it, it gets its own subsubcategory. Obviously, this is more soft sci-fi, since there is no method for time travel (and I’ll categorically state that there never will be). This breaks down further by method of time travel. Technological, mystical, or accidental?
• Technology – This is does not typically apply to sci-fi but think Tom Clancy. He describes how to build an atomic bomb, the workings of a submarine, and gets in the technical weeds. It’s science, right?

Fantasy also belongs to Speculative Fiction, which includes (but does not require) magic, non-earth settings, non-humans, and can just be clearly a different world than Earth, typically with limited technology. I’d actually class Star Wars here, though it has elements of sci-fi and fantasy. The Ring trilogy also belongs here.

Spiritual Fiction has a more faith-inspired bent. It can be entirely celestial, The Singers, or based on Earth with spiritual elements. My novel, Do Angels Still Fall? includes angels and the Trinity. The Left Behind series, and Lewis’ Space Trilogy belong here rather than in sci-fi. This isn’t limited to Christianity, of course, and can even extend to philosophy.

Alternate History is our final entry for the day. Harry Turtledove has made a comfortable living off of this. Change some element of history and how does it play out? The South won the Civil War, Kennedy wasn’t assassinated, Trump was elected President (oh, wait…)

There are some who would say the term Speculative Fiction is redundant, but it’s my crumpets and muffins.

Did I miss any?