The reason I specified space travel rather than sci-fi is that a large range of things can fall under sci-fi. Time Travel, alternate histories, cloning, mind control, alien invasions, mad science (BE Ray Guns and the like)...
Most of that stuff is covered by other tags; I wanted something more obviously "science fiction".
Similarly, the "fantasy" tag is specifically for episodes taking place in a fantasy world, rather than the broader definition of fantasy which includes magic wands, boob fairies, witches, magical transformations, etc...
I guess both tags are about things that take place away from Earth.