GMC 2001 Yukon XL Owner's Manual page 63

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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.
1-50
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A forward facing child seat (C
the child's body with the harness and also sometimes
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with surfaces such as T shaped or shelf
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E) provides restraint for
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like shields.