I'll explain the Sleeping Worlds/the realm of sleep. (This is a part of my "Explained" series of posts. To view my "Time Travel Explained" post, click here, to view my "Heartless, Nobodies, Unversed, and Dream Eaters Explained" post, click here, and to view my "The Braig/Xigbar Situation Explained" post, click here.) All the "realm of sleep" is, is dreams. Worlds can dream and people can dream. When a world falls to darkness/goes to the Realm of Darkness, they also fall asleep and they dream. The worlds we go to in Dream Drop Distance are dreams of worlds that were freed from darkness, but were not freed from their sleep. Yen Sid sent Sora and Riku back in time to Destiny Islands before it fell to darkness, so that they could sort of "ride" Destiny Islands into darkness, therefore ride it into sleep. Once they rode it into sleep and fell asleep, they unknowingly opened the first Sleeping Keyhole and were transported to the next Sleeping World/the first Sleeping World from the present-day that they need to save. When this happened, Riku unknowingly dove into Sora's dreams and became a Spirit Dream Eater to protect him from Nightmares. So, throughout the game, Sora is in the dreams of the worlds that fell to darkness, but didn't wake from their sleep, and Riku is in Sora's dreams. Things change when they get to The World That Never Was though. The World That Never Was is not a Sleeping World. It's a real world in the Realm Between. There's a cutscene where Yen Sid says that Sora and Riku will eventually end up out of the Sleeping Worlds and into a world that's in the Realm Between. That world was The World That Never Was. It's not a Sleeping World, but when Riku arrived there, he was still in the realm of sleep since he was still in Sora's dreams and has been the whole journey. After Riku defeats Ansem, Seeker of Darkness, he's brought back to the real world and out of the realm of sleep. When Sora arrived at The World That Never Was, he was in the real world and out of the dream. But, Young Xehanort dragged Sora back into the realm of sleep, but this time, he dragged Sora into Sora's own dreams instead of the world's dreams. Then, Xigbar dragged Sora into a second sleep. So, Sora's sleeping body was in Sora's own dreams which is why Riku saw it before he fought Anti Black Coat. Anyways, after Young Xehanort successfully puts Sora into a weird sleep coma by dragging him further and further into dreams, he brings him back to the real world and places him on one of the Organization XIII thrones. Riku still had his Dream Eaters and his Dream Eater powers in the real world of The World That Never Was because Sora was still asleep. He still had a weird Dream Eater connection to him. Nomura said The Mysterious Tower drifts around and doesn't stay in the same place. The Mysterious Tower used to be a part of a world called Symphony of Sorcery. It's drifted away from that world a long time ago though. The world "Symphony of Sorcery" is a Sleeping World. The reason The Mysterious Tower shows up in that world is because the dream is filling in the gaps, just like it fills in the gaps with characters. The Grid is a digital world inside of a computer in a world that we don't know the name of. The world that Sam Flynn is from. Just like how Space Paranoids is a digital world inside of a computer in Radiant Garden. Space Paranoids is a copy of The Grid made by Ansem the Wise. The reason Space Paranoids looks so different than The Grid though is because he copied it a long time ago. Space Paranoids is what The Grid used to look like. The Grid is not a Sleeping World, but the world it's attached to/the world it's a computer on, is a Sleeping World. Which is why Dream Eaters show up there. Country of the Musketeers is simply a world Mickey, Donald, and Goofy traveled to on their adventures. Minnie used to be a resident there, and she later moved to Disney Town to be with Mickey. Also, the Robed Figure was there on Destiny Islands in the first Kingdom Hearts so that he could mark the time traveling Sora with the Recusant's Sigil and so that Young Xehanort can time travel to where the Robed Figure is and follow Sora and Riku into the realm of sleep. So, now that Sora was marked with the Recusant's Sigil and now that Young Xehanort was in the same time period as Sora, he was able to teleport to wherever he was. Oh, and we now know that the official name for the realm of sleep is "Unchained." In Kingdom Hearts 3D: Dream Drop Distance, it was called many different things, it seemed to not have an official name. It was actually called "the realm of sleep" only once. So, yeah, it's the "Unchained" realm.