It's a tuning chip that turns ones 208bhp/258 lb-ft M274 into something more like the 240bhp/274lb-ft M274, which powers current 300- series MB products (think C300, E300, GLC300), vs the 250 series products (like the CLA/GLA250) which have the same tune as the Metris, albeit with the M270. But the only differences between the 270 and 274 are related to transverse or longitudinal mounting.
I think the difference between the two outputs is strictly a programming thing. However, I'd guess that driven aggressively a 208 would last longer than a 240, (that being true both with this box and from the factory).