GMC 2005 Sierra Denali Owner's Manual page 40

A forward-facing child seat (C-E) provides restraint for
the child's body with the harness and also sometimes
with surfaces such as T-shaped or shelf-like shields.
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A booster seat (F-G) is a child restraint designed to
improve the fit of the vehicle's safety belt system. Some
booster seats have a shoulder belt positioner, and some
high-back booster seats have a five-point harness. A
booster seat can also help a child to see out the window.