In the middle of things.
Like a bridge that touches two shores but doesn't belong to either of them.
So a story can touch the edges of two spheres, like between mortal and immortal, or fey and mundane. Or possibly be a smaller character's story, that takes place between the novels about The Great War.
Stories can also have a middling length. The general agreement is that short stories are usually under 6000 words, while novels officially start at 40,000.
But what about those tales that need a different length? That have more plot* than can be told in a short story, but aren't big enough to fill a novel?
One of the luxuries in this new world of publishing is that those stories don't have to automatically become trunk stories. They can be released out to the world, published on their own, instead of ignored and not written, or placed in a drawer, never to see the light of day.
This is a bundle of stories that take place in the middle of things. Some are about characters who are also in novel...