After Effects Nomenclature
An expression control is used to link a layer property through an expression. Expression controls can be animated which allows you to manipulate values within an expression, which in turn controls a layer’s properties. Expression controls can also be grouped together, often in a null layer, so that key properties can be controlled from the same location rather than having them spread out over many layers. You must have a layer selected in the timeline before you can apply an expression control, and they can be applied to any type of layer, including compositions. Following the links for each expression control below will clarify how these controls work.
Expression controls can be accessed via:
Effect > Expression Controls > [control name]
At the time of publication, there were 7 expression controls from which to choose: