Current Mercedes models adds diesel exhaust fluid and a selective catalytic reduction (SCR) on to the exhaust which they call BlueTec with Adblue. The 2010 C&C models will have a urea system but the 2500 and 3500 models will continue to use the traditional BlueTec system without urea. They are two seperate systems traditional BlueTec (the badge on the tailgate) and BlueTec with AdBlue (or exhaust fluid).
Traditional BlueTec - Diesel Oxidation Catalyst, DeNox catalytic converter, and DPF
The BlueTec with fluid - DOC, DPF and SCR
Either design controls nitrogen oxides and depending on which one will work the best to control the emissions that is the one used. This is the basic design but either can have a few different components added to meet emissions.