GM uses RPO codes to denote Retail Production Options.  There are several variations of the Duramax as the progresed over the years.  Since all are 6.6L displacement they are refered to by their RPO code.
Duramax RPOs used in 2500/3500 pickups
LB7 = 2001-2004, 300hp/520lb-ft
LLY = 2004.5-2005, 310hp/605lb-ft
LBZ = 2005-2007, 310hp/605lb-ft
LMM = 2007.5-2010, 365hp/660lb-ft, emission = DPF
LML = 2011-2016, 397hp/765lb-ft, emssions = DPF/DEF, known for CP4 pump failures.
L5P = 2017-2023, 445hp/910lb-ft
L5P Gen2 = 2024+, 470hp/975lb-ft

LB&/LLY/LBZs are sought after for being pre-emmissions (DPF/DEF).  Personally, I like our LMM that has 397,000 miles on it with original injectors.  The LB7s were known for needing injectors evey 100,000-150,000mi, which were labor intensive to replace.