Defects or Errors per Million Opportunities is a key measure for firms seeking improvement in their processes. Take a common item, like a baseball. Consider how many different attributes are important, such as the stitching, the diameter, the weight, density. Given these $4$ measurable attributes, you would conclude that there are $4$ opportunities, for each baseball, for things to go right or wrong. Therefore, if you found $1$ defect or error, on $2$ baseballs, out of your batch of $100$ baseballs, that would really be $2$ out of $400$ opportunities.