In the magic tab in your stats and journal menu, what does "Spell Effectiveness" mean?

Found and copied the following....

The Spell Effectiveness figure on the Magic screen indicates the actual percentage of how effective your spells are. The magnitude of a spell will be multiplied by this figure when you cast the spell. So if your Spell Effectiveness is 85%, an Ease Burden spell will only allow you to carry 42 more points, instead of the full 50 points. Your Spell Effectiveness will be reduced if you wear armor. There are two types of armor: light armor and heavy armor. It doesn't matter which one you actually use as far as your skill is concerned. Your overall Spell Effectiveness while in armor, either light or heavy, cannot exceed 95% regardless of your skill in that armor. 100% Spell Effectiveness can only be achieved if you are not wearing any armor (which is why NPC spellcasters always wear robes). Fatigue levels do not affect overall Spell Effectiveness.