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. The zero refers to scale's first dimension, which is width. If you replace the zero with a one (scale), Opacity will be controlled by height instead of width.
Along these lines, position means X, position means Y, and if you're working in 3D, position means Z. And, the same holds true for any positionable effect property, such as the Flare Center of a Lens Flare Effect.