Order Guide says:
On a 2500 chev:
U01 Lamps, Smoked Amber roof marker, (LED)
1 - Included with (ANQ) Alaskan Snow Plow Special Edition. Not available with (YF5) California state emissions requirements.
2 - Not available with (YF5) California state emissions requirements.
On a 3500chev:
U01 Lamps, Smoked Amber roof marker, (LED)
1 - Included with (ANQ) Alaskan Snow Plow Special Edition. Standard with dual rear wheels. Not available with (YF5) California state emissions requirements on single rear wheels.
2 - Standard with dual rear wheels. Not available with (YF5) California state emissions requirements on single rear wheels.
On the GMC version, U01 is included with some of the convenience packages.
This is 3500 sierra:
U01 Lamps, Smoked Amber roof marker, (LED)
Base and Denali- Standard with dual rear wheels. Not available with (YF5) California state emissions requirements on single rear wheels.
SLE - Standard with dual rear wheels. Not available with (YF5) California state emissions requirements on single rear wheels. Included with (PCQ) SLE Convenience Package.
SLT - Standard with dual rear wheels. Not available with (YF5) California state emissions requirements on single rear wheels. Included with (PCY) SLT Convenience Package.
AT4 - Not available with (YF5) California state emissions requirements. Included with (PDO) AT4 Preferred Package.
BUT, YF5 emissions overrides the 'included with' information.
Example for 3500 sierra:
PCY SLT Convenience Package, includes (A50) bucket seats with (D07) center console, (KQV) ventilated front seats, (A48) rear sliding power window, (UG1) Universal Home Remote, (MCR) USB Ports (Inside Console with bucket seats), (MCZ) 2 USB ports with auxiliary input and (KA6) rear heated seats; Single rear wheel models also include (U01) Roof Marker Lamps
SLT - Included with (PDT) SLT Premium Plus Package and (PDW) Texas Edition SLT Premium Plus Package. Note (U01) Roof Marker Lamps are not available with (YF5) California state emissions requirements on single rear wheels.