GMC Envoy Owner's Manual page 43

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.
1-36
CAUTION:
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Each top tether bracket is designed to anchor
only one child restraint. Attaching more than
one child restraint to a single bracket could
cause the anchor to come loose or even break
during a crash. A child or others could be
injured if this happens. To help prevent injury
to people and damage to your vehicle, attach
only one child restraint per bracket.
Anchor the top strap to one of the following anchor
points. Be sure to use an anchor point located on the
same side of the vehicle as the seating position
where the child restraint will be placed.
Raise the head restraint and route the top strap under
it. See Head Restraints on page 1-5.
Once you have the top strap anchored, you will be
ready to secure the child restraint itself. Tighten the top
strap when and as the child restraint manufacturer's
instructions say.