It's a code that Dell puts on their more premium products that makes any dealings with warranty a lot simpler. QC may or may not be checking for dead pixels during calibration - it's definitely not a given since the latter process can be largely automated, but even if they do there still seems...
Does the U2515H have a service tag? Dell has previously stated that 24" and smaller displays don't, but that was written during the time there was nothing between 24" and 27".