From my 2004 Tahoe manual...
Luggage Carrier
If you have the luggage carrier, you can load things on
top of your vehicle.
The luggage carrier has slats and siderails attached to
the roof and crossrails which can be moved back
and forth to help secure cargo. Tie the load to the
siderails or siderail supports.
Notice: Loading cargo on the luggage carrier that
weighs more than 200 lbs. (918 kg) or hangs
over the rear or sides of the vehicle may damage
your vehicle. Load cargo so that it rests on the slats
as far forward as possible and against the side
rails, making sure to fasten it securely.
Don’t exceed the maximum vehicle capacity when
loading your vehicle. For more information on vehicle
capacity and loading, see ′′Loading Your Vehicle′′.
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To prevent damage or loss of cargo as you’re leaving,
check now and then to make sure the luggage and
cargo are still securely fastened.
Be sure the cargo is properly loaded.
• If small heavy objects are placed on the roof, place
the load in the area over the rear wheels (behind
the rear side door on Suburban Models). If you
need to, cut a piece of 3/8 inch plywood to fit inside
the crossrails and siderails to spread the load.
If plywood is used, tie it to the siderail supports.
• Tie the load to the crossrails or the siderail supports.
Use the crossrails only to keep the load from sliding.
To move the crossrails, pull out on the latch release
handle at each end. Slide the crossrail to the desired
position balancing the force side to side. Push the
release handle back into the latched position and
slide the crossrail back and forth slightly to be sure
the latch snaps securely into place.
• If you need to carry long items, move the crossrails
as far apart as they will go. Tie the load to the
crossrails and the siderails or siderail supports.
Also tie the load to the bumpers. Do not tie the load
so tightly that the crossrails or siderails are damaged.
• After moving a crossrail, be sure it is securely locked
into the siderail.
Your vehicle has a Center High-Mounted Stoplamp
(CHMSL) located above the glass or above the rear
load doors.
If items are loaded on the roof of the vehicle, care
should be taken not to block or damage the CHMSL unit.
If your vehicle has the Z71 option your vehicle’s
luggage carrier has a roller crossrail at the rear of
the vehicle.
To load cargo, place cargo on the roller and roll the
cargo forward onto the two front crossrails.
To move the crossrails do the following:
1. Loosen the thumbscrew at each end.
2. Move crossrail to desired position.
3. Tighten the thumbscrews.
Make sure the crossrail is secured.