GMC 2001 Sierra - Manual (Page 64)

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Wherever you install it, be sure to secure the child
restraint properly.
Keep in mind that an unsecured child restraint can move
around in a collision or sudden stop and injure people in
the vehicle. Be sure to properly secure any child
restraint in your vehicle
-- even when no child is in it.
Top Strap
Some child restraints have a top strap, or “top tether.”
It can help restrain the child restraint during a collision.
For it to work, a top strap must be properly anchored
to the vehicle.
If the child restraint does not have a top strap, one
can be obtained, in kit form, for many child restraints.
Ask the child restraint manufacturer whether or not a
kit is available.
In Canada, the law requires that forward - facing
child restraints have a top strap, and that the strap be
anchored. In the United States, some child restraints also
have a top strap. If your child restraint has a top strap,
it should be anchored.