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- In-universe perspective
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Re-write for Room of Requirement
1. Under History of Use
- Room of Hidden Things Done.
- Dumbledore's Army room (1995-1996 school year)
- Dumbledore's Army room (1997-1998 school year)
- Chamberpot room (mentioned by Dumbledore in GoF) Done.
- House-elf sobriety room (used by Dobby for Winky) Done.
- Cleaning supplies closet (Dobby mentions Filch finding it like this once) Done.
- Broom cupboard (Fred and George mention hiding from Filch in here) Done.
3. Source the whole article
Room of Requirement
The Room of Requirement is a secret room in Hogwarts that only appears when a person is in great need of it. The room then transforms itself into whatever the witch or wizard needs it to be at that moment, although there are some limitations. For example, it cannot create food, as that is one of the five Principal Exceptions to Gamp's Law of Elemental Transfiguration.  It is believed that the room is Unplottable, as it does not appear on the Marauder's Map, nor do its occupants, although this could simply be because James Potter, Remus Lupin, Sirius Black, and Peter Pettigrew never found the room.
The room is located on the seventh floor of Hogwarts castle, and has a hidden entrance opposite the tapestry depicting the attempt of Barnabas the Barmy to teach trolls ballet. To enter the room, you must walk past it three times thinking about what you need. A door will appear, which leads to the room, furnished to take care of your needs. 
It is advised that the users of this room be very specific about what they are looking for, as it has been shown that other people can enter the room and see what the occupant is doing if they know how the room is being used. The Inquisitorial Squad found the hideout of the D.A. in 1996 when they found out that the D.A. was in the room at the time, and knew how to get in, due to Marietta Edgecombe telling Dolores Umbridge.  Neville Longbottom used this knowledge to create his hideout, as he specifically asked that the hideout be made unavailable to those who supported the Carrows. 
History of Use
The first mention of this room was in 1994, when Albus Dumbledore told Igor Karkaroff of his discovery of a room full of chamber pots when he needed to go to the toilet. However, he was unable to find the room again later. 
- Hermione Granger: "You've done it, Neville! You've found the Room of Requirement... It's also known as the Come and Go Room. It can only be used by someone who really needs it. And it's always equipped for the seeker's needs."
- Ron Weasley: "Right. Say you really needed the toilet?"
- Hermione Granger: "Charming, Ronald. But yes."
- — Hermione to D.A. on their first meeting in 1995[src]
Harry and Dumbledore's Army used the room as their headquarters and practice area. Among the items provided by the room for the D.A. were Dark Detectors and shelves of helpful books such as A Compendium of Common Curses and their Counter-Actions, The Dark Arts Outsmarted, Self-Defensive Spellwork, and Jinxes for the Jinxed. They were eventually discovered by the Inquisitorial Squad.
House-elf Sobriety Room
When Dobby was in need of a place to care for Winky after she had gotten drunk on Butterbeer, he found this room perfectly suited for his needs. The room was filled with antidotes to butterbeer and an elf-sized bed for Winky to sleep off her inebriation. 
Room of Hidden Things
Harry used this room again in his sixth year to hide his copy of the Half-Blood Prince's Potions textbook from Professor Snape. He found that many other students from past years had used the room to hide a multitude of objects, including the Vanishing Cabinet. Inside the cabinet in which he hid the book, Harry found a cage that contained the skeleton of something that had 5 legs.
Also in Harry's sixth year, Professor Trelawney attempted to use the room to hide her sherry bottles, but Draco Malfoy threw her out. Draco was using the room to repair the Vanishing Cabinet so he could smuggle Death Eaters into the castle via a matching Vanishing Cabinet in Borgin and Burkes.
Harry also shared his first kiss with Ginny Weasley in this room.
Objects Hidden Within
Harry Potter saw many things that students had hidden in the room, including:
- Broken and damaged furniture, perhaps hidden to hide mishandled magic
- Thousands and thousands of books, probably banned, graffitied, or stolen
- Flying catapults
- Fanged Frisbees
- Chipped bottles of congealed potions,
- Corked bottles whose contents still shimmered evilly
- What looked like dragon eggshells
- Several rusting swords
- A heavy, bloodstained axe
- An enormous stuffed troll
- The broken Vanishing Cabinet that Montague had gotten lost in during the previous school year
- Hats, jewels, and cloaks
- A cage with a quintaped skeleton
- The lost diadem of Ravenclaw, made into a Horcrux
- The Half-Blood Prince's copy of Advanced Potion-Making
- A chipped old bust of an ugly warlock
In the 1997–1998 school year, Neville Longbottom and several other students used the room as a hideout to escape the wrath of the new Death Eater professors, the Carrows. The problem was the lack of food, as it was one of the exceptions to Gamp's Law of Elemental Transfiguration. So the hideout adapted by adding an eighth passage into Hogsmeade, connected to the Hog's Head pub, whose owner, Aberforth provided food. Also, this advantage allowed members of the Order of the Phoenix to enter the school to help fight off Lord Voldemort's attack, without setting off the Caterwauling Charm on the village.
Later that same year, during the Battle of Hogwarts, it was used as an evacuation route to move the students away from the dangers that followed. Later, Harry realised that Rowena Ravenclaw's Diadem, one of Voldemort's Horcruxes, was hidden in the Room of Hidden Things, as he has seen it in his sixth year while hiding his Potions book and had even picked it up without realizing what it was. He, Hermione and Ron managed to find it, but were ambushed by Draco Malfoy, Vincent Crabbe and Gregory Goyle. Crabbe let loose cursed fire that destroyed the Horcrux and many other objects, killing himself in the process.
Behind the Scenes
- In the fifth film, Neville Longbottom found the Room of Requirement, since Dobby was cut from the film.
- In the sixth film, Harry and Ginny had their first kiss in the Room of Hidden Things, instead of the Gryffindor common room.
- In the eighth film, Gregory Goyle casts the Fiendfyre curse in the Room of Hidden Things, destroying the diadem and killing himself in the process, since Vincent Crabbe was cut from the two-part film.
- Harry Potter and the Goblet of Fire (First mentioned) (Not mentioned by name)
- Harry Potter and the Order of the Phoenix (First appearance) (First identified as Quidditch Lover/parchment)
- Harry Potter and the Order of the Phoenix (film)
- Harry Potter and the Order of the Phoenix (video game)
- Harry Potter and the Half-Blood Prince
- Harry Potter and the Half-Blood Prince (film)
- Harry Potter and the Half-Blood Prince (video game)
- Harry Potter and the Deathly Hallows
- Harry Potter and the Deathly Hallows: Part II