This is a draft "security hardness scale", desgigned to somewhat roughly quantify the level of effort of a penetration test -- since simply measuing "how many vulns did you find" is a terrible measurement of success. The goal is the measure the "hardness" of the system under test in a way that's a bit quantitative.
The result is a score from 1-10. The scale is inspired by to the Mohs Hardness Scale in that it's simply an ordinal scale, not an absolute one. That is, the "gap" between 3 and 4 doesn't have to be the same "difficulty increase" as the gap between 5 and 6. It's simply a way of rating that one pentest was "harder" than another. (This is in lieu of being able measuing "hardness" in any truely quantitative way).