“Decamarks,” isn’t just one item, it’s a placeholder for any item not defined by the game. Item ids take up four bytes in the game’s code, meaning that there are 65,536 slots available for different items. Gen 3 uses less than 400 of these slots and Gen 4 uses less than 600, so the vast majority of them are unused, and will be displayed as decamarks. I didn’t test every single one, but I tested a good handful from Emerald to both Platinum and HeartGold and found no difference between games.
While all decamarks look and act the same in Gen 3 games, I’ve found two variants that behave differently in terms of migration. The first variant are items that are undefined in Gen 3, but would be valid in Gen 4. A Pokémon holding these items are blocked from migration entirely. The second variant are items that are undefined in both Gen 3 and 4. A Pokémon holding one of these items can migrate, and will appear to be holding something in the migration menu, but once in the PC, it will be holding nothing.
