GMC Yukon Denalid Owner's Manual page 146

PARK (P): This position locks your rear wheels. It
is the best position to use when you start your
engine because your vehicle cannot move easily.
When parked on a hill, especially when the
vehicle has a heavy load, you may notice an
increase in the effort to shift out of Park.
See Torque Lock (Automatic Transmission) under
Shifting Into Park (P) on page 153 for more
information.
146
CAUTION:
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It is dangerous to get out of your vehicle
if the shift lever is not fully in PARK (P)
with the parking brake firmly set. Your
vehicle can roll.
Do not leave your vehicle when the engine
is running unless you have to. If you have
left the engine running, the vehicle can
move suddenly. You or others could be
injured. To be sure your vehicle will not
move, even when you are on fairly level
ground, always set your parking brake
and move the shift lever to PARK (P). See
Shifting Into Park (P) on page 153. If you
are pulling a trailer, see Towing a Trailer
on page 400.