Yes, it's a literal summer-long challenge. I like to give people as much time as possible, LOL - and original fic is more than welcome! =) You can have a look at last year's round-up if you'd like; this is very much a low-pressure muse-boosting sort of thing.

do the prompts have to appear in the stories or can they be "just" inspiration or metaphorically used or story/chapter titles etc.?

The word doesn't have to appear in the story, no :)