Initiative (I) reflects a character's speed of thought, whether he is quick-witted or sluggish.
In close combat, I determines who acts first. This is expressed as a percentage of 01-100%.
I reflects speed of thought and action. It is used to test actions requiring mental alertness rather than physical skill, to determine whether a character spots some small or unusual circumstance (such as a tear in a man's clothing or a dislodged picture on the wall), and in cases where the character must react to something quickly, such as dodging a falling rock.