One Shot meaning you write the story all in one go and don't update it. The other is Fan Fiction or fan fic for short, this one you update and it can be as long as you want, some go up to 100's of chapters according to you how long you want to stretch your story, and it's open to anyone to write on the forum, if you are writing a fan fic, every time you update you need to change the title to say what number update it is and on what page.
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