Chris and Trish Meyer posted the 4th of a 12-part series, Deeper Modes of Expression, based on an extra in their book, Creating Motion Graphics with After Effects. The fourth installment is Deeper Modes of Expression, Part 4: Space Conversions:
"As you work with Position values and expressions, you will run into the problem of what the Position value really means: Is it a position relative to the layer (such as an effect point), is it a position relative to a parent layer, or is it a position relative to the overall comp (sometimes referred to as “world space”)? Fortunately, After Effects has a number of layer space transform methods designed to help you move between these different definitions of Position. These are described in the online Help (press F1 to open). Most are straightforward, but there are a few useful transformations that need some extra thought."