SP Class is the collective name given to the generic enemy classes that appear in Part 4 of Fire Emblem: Radiant Dawn. Even though they resemble regular 2nd Tier classes, they possess qualities halfway between 2nd Tier and 3rd Tier classes. Their class names and appearance match 2nd Tier classes, but their awarded experience and Skill capacities match 3rd Tier classes. On the other hand, their base stats, stat caps and weapon proficiencies are around halfway between the two tiers. One noteworthy trait of the SP generals is that they can use Lances, Axes and Swords.
The name SP is derived from the game data. As for what it means, it is unknown, although "special" and "super" may come to mind.