What specific details make a story a mystery? Does it have to be a crime investigation or similar to that?
No, it doesn't have to be a crime. There has to be something major at stake, but you could, for example, base a mystery on who is the proper heir to a fortune. In fact, there is an episode of a radio series in which the insurance investigator main character has to locate a witness to a fatal automobile accident. The two victims named each other as primary beneficiaries, but each named a different secondary beneficiary. The witness had reported the accident and said one of them was still alive at the time, but didn't say which one. So one beneficiary or the other was due the combined insurance benefit, depending on which occupant outlived the other, and therefore the anonymous witness had to be located and questioned. No crime was necessary to make this a good, taut mystery. All you need is a puzzle to be solved, and often more important, interesting characters to delve into to collect the clues that lead to a solution.
Thanks Cogito, I think I have a short mystery story to post based on this, and if I have the credentials I will do that. If the mods determine it isn't a mystery hopefully they will simply move it to fiction.
Keep in mind that the genre classifications i our Writing Workshop are only suggestions. Members will apply criteria they believe appropriate to the genre, but most recommendations tend not to be all that genre-specif7c anyway. Word choice or sentence structure suggestions, for example, are cross-genre. So you don't have to meet strict criteria for the section choice.