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.
Perfect tense is a category of verb tense used to describe completed actions. It covers the past perfect tense, the present perfect tense, and the future perfect tense. It is sometimes called the complete tense.