GMC Yukon Owner's Manual page 128

In addition, the vehicle has dual-stage frontal airbags.
Dual-stage airbags adjust the restraint according to
crash severity. The vehicle has electronic frontal
sensors, which help the sensing system distinguish
between a moderate frontal impact and a more severe
frontal impact. For moderate frontal impacts, dual-stage
airbags inflate at a level less than full deployment.
For more severe frontal impacts, full deployment
occurs.
The vehicle has a seat position sensor. Vehicles with
dual stage airbags also have seat position sensors
which enable the sensing system to monitor the position
of the driver seat.
The seat position sensor provides information that is
used to determine if the airbags should deploy at a
reduced level or at full deployment
The vehicle may or may not have seat mounted side
impact airbags. The vehicle has roof-rail airbags.
See Airbag System on page 2 83. Seat mounted side
impact airbags and roof-rail airbags are intended to
inflate in moderate to severe side crashes. In addition,
these roof-rail airbags are intended to inflate during a
2-90
rollover or in a severe frontal impact. Seat mounted side
impact airbags and roof-rail airbags will inflate if the
crash severity is above the system's designed threshold
level. The threshold level can vary with specific vehicle
design.
Roof-rail airbags are not intended to inflate in rear
impacts. A seat mounted side impact airbag is intended
to deploy on the side of the veicle that is struck. Both
roof-rail airbags will deploy when either side of the
vehicle is struck or if the sensing system predicts that
the vehicle is about to roll over, or in a severe frontal
impact.
In any particular crash, no one can say whether an
airbag should have inflated simply because of the
damage to a vehicle or because of what the repair
costs were. For frontal airbags, inflation is determined
by what the vehicle hits, the angle of the impact, and
how quickly the vehicle slows down. For seat mounted
side impact and roof-rail airbags, deployment is
determined by the location and severity of the side
impact. In a rollover event, roof-rail airbag deployment
is determined by the direction of the roll.