Technically, all fiction is fantasy, so every type of fiction is a subgenre.
I would agree with that, but be even more precise; all fiction is alternate history
; there is only one History, but infinitely many Stories!
Someone earlier made the point better than I, but to use my own example: a story where people travel faster than light is a science fiction story. If the story could be told without the characters being able to travel faster than light, that's a bad
sci-fi story. If the story not only required that the characters be able to travel faster than light, but also showed how the characters were altered by this fact, and their actions, words and thoughts provoked an interesting comment about people today, that would be a good sci-fi story; perhaps today, a great