From the Super Mario Wiki
In Mario vs. Donkey Kong: Minis March Again!, there is a Special Level on each of the regular floors and their Plus Mode couterparts. It is unlocked by collecting all nine lettered Mini Mario Cards on its respective floor. Special Levels are considerably bigger than the other levels and often feature more enemies and items. The Mini Mario Cards found in Special Levels display a crown instead of a letter.
Special Levels contain the "SP" abbreviation in their designation. The following are all Special Levels in the game: