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[glsurratt]

Maybe this will explain it - see the second part here for "Engine Idle Shutdown ...63T":

FEDERAL 49 STATE EMISSIONS SYSTEM
Forced on 6.4L diesel without 63T Engine Idle Shutdown
or 63E Engine Idle Shutdown Exemption
50 STATE EMISSIONS SYSTEM .......................................... 425
Standard on all gas engine units. Forced on 6.4L diesel
with 63T Engine Idle Shutdown or 63E Engine Idle
Shutdown Exemption.
ENGINE IDLE SHUTDOWN .................................................. 63T
Required on 6.4L Diesel with Automatic Transmission in
the following states: CA, CT, DE, GA, ME, NJ, NY, NC,
PA. Optional on 6.4L Diesel with Automatic Transmission
in all other states. Not available with Manual
Transmission, 76R Reverse Vehicle Aid Sensor, or
Navigation Radio. Can be deleted for vehicles ordered by
dealers in CA, CT, DE, GA, ME, NJ, NY, NC, and PA for
delivery outside of those states. California and affected
green states require non-exempted vehicles with diesel
engines rated over 14K GVW to incorporate an idle
shutdown system to prevent extended engine idle
periods. After a predetermined period, the engine PCM
automatically shuts down the engine and triggers the
accessory module to shutdown power to the accessories
to minimize battery drain.
ENGINE IDLE SHUTDOWN EXEMPTION ............................ 63E
Optional on 6.4L Diesel. Vehicles exempt nationwide
from requiring the Idle Engine Shutdown System include
buses, recreational vehicles, emergency vehicles and
military tactical vehicles. Requires certification that the
state exemption requirements are met.
IDLE ENGINE SHUTDOWN NOT REQUIRED ..................... 633
Optional on 6.4L Diesel. Vehicles that do not qualify for
63E exemption and are not intended for registration in
CA, CT, DE, GA, ME, NJ, NY, NC, or PA are not
required to be equipped with idle engine shutdown.
Dealers in affected states can select this code if the
vehicle is not intended for registration in CA, CT, DE,
GA, ME, NJ, NY, NC, or PA or if the vehicle qualifies for
any state-specific exemptions from California Emissions
requirements in CT, DE, GA, ME, NJ, NY, NC or PA.