4) +1 INT is equivalent to +1 MACC when the difference between the caster's and target's INT is at or below +10. +1 INT is equivalent to +0.5 MACC when the difference is above +10. It is currently unknown if there is another "tier".
This is meant in the context of spells that are based on an INT to INT check, which is true for say Elemental Magic. Other moves like Paralyze rely on a MND to MND check. Even still more complex, the stats of the mob and player don't always have to be the same. Certain Weapon Skills such as COR's Leaden Salute are based on an INT to AGI check. It depends on the individual move in question, but the general concept is likely to be the same.