The Union Room allows players the interact directly with other players. It is the successor to the Cable Club that was present in Generations I and II. Other Trainers appear in the Union Room as representations of nearby players who have also entered the room. In Generation III (more specifically, Pokémon FireRed, LeafGreen, and Emerald) the Union Room requires the GBA Wireless Adapter to be attached to the Game Boy Advance to enter, and in Generation IV it can be entered using the DS's native Wi-Fi ad hoc support. The player can exit the Union Room by stepping on the warp tile, which will send the player back to the entrance of the Union Room in the Pokémon Center.
Inside the Union Room, Trainers may:
- Chat in a group of 2-5 Trainers.
- View the Trainer Card of another Trainer.
- Enter two Pokémon up to level 30 for one-on-one battle.
- Register a Pokémon for trade, or trade for a registered Pokémon (Generation III)
- Trade Pokémon (Generation IV)
- Mix Records with up to 5 people (Generation IV)
- Draw (Generation IV)
- Spin Trade (Pokémon Platinum, HeartGold and SoulSilver)
Despite the high potential of the Union Room and the variety of activities there, they are often empty, except if a certain time is scheduled by a group of friends or relatives. This is because once in a Union Room, only specific Union Room actions can be carried out.
The receptionist in the Union Room will talk to the player if no other Trainers are present in the Union Room at the time. If the player selects "Converse" in Generation IV with the most recent message from the player being the default, she will say, "Welcome to the Union Room! I'm the only one here right now, but please do bring your friends for some fun and excitement! If you have time on your hands, I'm also here to chat. You see, I have a lot of time on my hands..."
While in the Union Room, every player is assigned a random Trainer class, such as Ace Trainer or Bug Catcher, to help differentiate between players. In Diamond, Pearl, and Platinum, this can be changed by talking to the Psychic in the Oreburgh City Pokémon Center. In HeartGold and SoulSilver, this can be changed by talking to the Ace Trainer in the Violet City Pokémon Center. In Black and White, this can be customized on the player's Trainer Card.
This lets each player see the other's Trainer Card. If a Sinnoh player views a Johto Trainer Card, there won't be any Badges obtained, due to the difference in region. Similarly, if a Johto player views a Sinnoh Trainer Card, there won't be any Badges obtained.
- Main article: Mixing records
When performing this action, one can mix records with their friends. There are different effects in the two generations.
In Generation III
When mixing records, shows on TV will now tell the player what the player who they mixed records with had done prior to the record mixing, such as what the player bought at the Poké Mart or the last Pokémon the player caught by fishing. Also, the Trainers at the Pokémon Trainer's Fan Club will start praising the recorded player, instead of the original player.
In Generation IV
Players can mix records with up to 4 others at a time. Mixing records will share dress-up data and group data between the participants. All participants will be able to view the dressed-up Pokémon of the other participants in their own games. They will be able to join the other participants' groups.
Mixing records allows the joining of other groups. There are many in-game similarities between two players in the same group:
- All of the player's daily Feebas spots in Mt. Coronet are synchronized.
- The Pokémon of the day at the Great Marsh will also be the same for each of the players.
- The slot machine odds are the same at the Game Corner.
- The group rankings and records on the 3rd floor of Jubilife TV are shown, comparing each group member.
- Each member's daily swarming Pokémon is the same.
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In Generation V
Many of the controls now on the touch screen. The left button allows the player to post messages and view previous messages. These messages, however, are words from a fixed word bank inputted into a fixed sentence. The first one in from the left will challenge everyone in the room by placing a message on the message board telling others to tap it if they wish to battle the challenger. The middle will do the same but offer a trade instead of challenge on the bottom screen. The same for the button second from the right, which invites another player to a drawing session. The very left does the same, but offers an Egg spin trade.
- Main article: Trade
The Union Room is usually used to trade Pokémon between two players within a short distance of each other. When a traded Pokémon comes to the player's cartridge, it will get 1.5x the experience the player's original Pokémon would get. The only drawback, however, is that the traded Pokémon will only listen to the player if he or she have enough Badges. Otherwise, the Pokémon may not obey the new Trainer. Trading is often used to get version exclusives, such as Magmar and Electabuzz or Stunky and Glameow.
When trading, the Pokémon offered to trade is selected by its owner. When two Pokémon are selected, the game asks each player whether or not they want to trade. If both players select yes, then the trade will commence. It is also possible to look at the summary of each Pokémon when the trade option is selected.
- If two Generation IV games in different languages are connected using the Union Room, the other language's word for "hello" will be displayed when the character from the other language is spoken to. For example, an English copy connected to a Japanese copy will display "こんにちは," while the Japanese copy it is connected to will display "Hello."