@LifeSucks619 hey there! It depends on whether fandom knowledge is required to understand your work! If you had followed the main storyline of the anime very closely, and if your readers will be lost if they do not know the universe, then its headcanon. If its an adaptation or original story spinned from the anime, and that non fans can still appreciate it without fandom knowledge, then it's open ff. If it's a one shot, short story or scenario based, it's under short story or one shots. If it is a quote book or song book or any random stuff that does not fit into any of the above, it's under random.