The output is vector interpolated to the scale of the current plot. When used in a source, the output of the source is the interpolated vector, or the initial or final value for points off the ends of the original scale.
For example, say an amplifier produces vector v(1) (an output) in plot tran1. One desires to apply this as input to another circuit. This is achieved with a source specification like that shown in the example above. This works in ordinary vector expressions as well.