GMC Sierra Denali 2009 Owner's Manual page 46

Child Restraint Systems
The harness system holds the infant in place and, in a
crash, acts to keep the infant positioned in the
restraint.
1-40
A rear-facing infant
seat (A) provides restraint
with the seating surface
against the back of
the infant.
A forward-facing child
seat (B) provides restraint
for the child's body
with the harness.
A booster seat (C-D) is a child restraint designed to
improve the fit of the vehicle's safety belt system.
A booster seat can also help a child to see out the
window.