GMC 2004 Yukon XL Owner's Manual page 58

A rear-facing infant seat (B) provides restraint with the
seating surface against the back of the infant. The
harness system holds the infant in place and, in a crash,
acts to keep the infant positioned in the restraint.
1-52
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.