The Stat change scale is basically a scale indicating how much a character or enemy's stats are affected by stat changes (such as DefenseUp). Basically, each use of Defense or Offense Up gives +1 to the "modifier" on the stat, whereas Defense or Offense Down gives -1 to the modifier. The modifiers affect the actual stat differently depending on how many there are. The first offense/defense up/down gives a 10% boost/penalty to the stat, the next two give 5% boosts/penalties, and the fourth and subsequent have no effect (they fail).