>Mixing records is a way to share information with other players, which is able to be done in the Record Mixer. The Record Mixer is a room in the Pokémon Center that allows players to mix records with up to four players. Mixing records allows the players to swap their data from other players. Some of the things that are done are just meant for entertainment, and some have an effect on what the player can do with their Pokémon in the game. The Record Mixer and the record mixing feature was introduced in Generation III as a special feature using the enhanced multiplayer mode on the Game Boy Advance. The Record Room is located in any Pokémon Center in the second floor on the far left in Generation III and inside the Union Room in Generation IV.
-bulbapedia
Effects in gen4
Television shows
There are a couple of shows that use the Record Mixer featured in Pokémon Diamond, Pearl, and Platinum Versions, including the Trend Tracker show, Trainer Research, and Sinnoh Now, that informs players of stats about how many berries picked on a certain day, battles fought, and Contest information about the recorded player. They also show interviews about recorded players along with trendy phrases set up by the recorded player.
Feebas factor
When the player mixes records, all the players who mix records will get the same positioning of the Feebas in Mt. Coronet. The four tiles where Feebas can be obtained will change to where the Player 1's slot tiles are in the game. This can make it easier to obtain Feebas if all four players search at the same time. The Feebas tiles do not stay in the same spot however; they change the next day.
Great Marsh
Mixing records also changes the special Pokémon of the day in the Great Marsh so all recorded players have the same Pokémon on the same day. Each day, a special Pokémon will change in the Great Marsh; therefore, to get a new set, players must wait a day. These Pokémon will appear randomly in one of the six areas, some may not appear for a while, while others appear several days in a row or in multiple areas on the same day.
Game Corner
When the player mixes records, all the players who mix records will strangely equalize the chances of winning in the Veilstone Game Corner.
Jubilife TV
When players mix records, if they have used the dressing up studio on the second floor of Jubilife TV, other players' photos will be displayed alongside the player's.
Safari Zone
In Pokémon HeartGold and SoulSilver, players that mix records will have the same customization of the six panels in the Safari Zone.
Basically anything that shows your information eg Safari Zone will also show your friends.
From Bulbapedia
