GMC Sierra Denali 2006 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.
1-34
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.