Remember fractional distillation?  A technique to separate mixtures that has components of different boiling points.

fractional_distillation

So how does this connect with testing software? Well , a system can be thought of as a mixture of various types of defects. And so to uncover these defects, can we separate these types of defects on the basis of certain properties and uncover them optimally? This is the inspiration behind “Quality levels” in HBT, where instead of unit/integration/system testing levels, HBT proposes NINE quality levels where specific types of defects are attempted to be uncovered at each level, similar to different liquids being siphoned off at each of the levels in the fractionating column.