Jargon, although technically in the industry, it's called "six sigma", because people who designate industry jargon are hacks that want to make their claims sound better than they are. =P

It's a reference to "acceptable defect rates" that most factories have to conform to. Factory management says "we can only permit X% of defects to leave the factory", and if your defect rate is higher, the workers (or hopefully, instead, the plant management) get dinged. "Sigma" is a reference to the statistical concept of Standard Deviation, three sigma being three standard deviations, which on a normal …