Hello Toby,
I am Venom, an author who has reached Level Three so far. Therefore, I am not the one in charge here. It is possible that your post wasn't noticed because of technical and/or organisational difficulties. So I will take the liberty to answer it:
"Assignment" means "writing" - stories, of course, but at the beginning there also can be something like "Tell us the reasons why you want to be an author". But all assignments will be of literary nature, if this was your question. There won't be anything like: "Get five nudes of Miss Piggy."
The stories I wrote for the Writer's Block were quite long - it took about four weeks each (maybe ten to fifteen hours of pure writing). But there is absolutely no need for such extended narrations. Content counts.
An assinment could look like "Write a story about a girl meeting her kinky aunt" as well as "Create a story including the words green-coloured, shutter and sulphur".
You upload your story, and the Level's tutor will edit it and give you tips for improving it. Once you have acquired sufficient skills, you will enter the next Level.
Unlike my narrations in the Library, my stories for the Writer's Block were generally not erotic. One for example was about a car terrorising people at a petrol station, an other was classic Fantasy with swords and knights. Not erotic or BDSMesque at all.
To your question "Where can I learn more about what's expected of an author?"
I am sure, Level One is exactly the place for it.