One of the two of:
a. The ability to reduce the levels of a spell
b. the ability to unlearn/forget a spell
either one will work. this is for when you were too aggressive before using cheats and need to lower level. such as for example to do a quest that requires you to level up telekinesis. that spell has a level cap, so you could have already reached the level cap before getting the quest. which beaks the save in question.