Another Dimension

When you add an expression to a one-dimensional property, such as Opacity and Pickwhip a two-dimensional property, such as Scale, After Effects will add a bracketed number to the end of the expression: scale[0]. The zero refers to scale's first dimension, which is width. If you replace the zero with a one (scale[1]), Opacity will be controlled by height instead of width.

Along these lines, position[0] means X, position[1] means Y, and if you're working in 3D, position[2] means Z. And, the same holds true for any positionable effect property, such as the Flare Center of a Lens Flare Effect.