If an artist wanted to create a relief in which a tree appeared further away than a park bench, carving the park bench more deeply (in high relief) would achieve this effect.

The statement is true. By definition relief in sculpting refers to the elements that are placed on top of a flat surface. Things that are "low" only stand lightly out from the background making it seem as if they are part of the background while those in "high" are carved further from the background making them seem like they are on top of the background.