Placeholder data is data relating to generic entries among other entries which for some reason are made valid. This data may be added so that it can be altered later (example: perhaps the data for the 25 "?" in Pokémon Ruby, Sapphire and Pokémon Emerald and Pokémon FireRed and LeafGreen), or for consistency (example: perhaps Arceus' ??? form in Generation IV).
Placeholder data may give information about the development of a Pokémon game, for example, in the Generation III core series games there is a placeholder item with an index number of hex:010B. This is directly after Contest Pass and directly before Wailmer Pail, suggesting an extra key item may have been considered.
Examples
Arceus's "???" form in Generation IV.
?'s picture and name.
Missingno.'s name and placeholder Pokédex data and how it permeates 39 entries in between the list of valid Pokémon.
???????? items.