Clarity considers a stone's clarity characteristics, both internally and externally. These range from Flawless, Internally Flawless, VVS1, VVS2, VS1, VS2, SI1, SI2, and I1.
A lab report provides a loose guideline for these characteristics but does not necessarily factor in the overall appearance.
For example, picture an SI diamond with tiny black graphite inclusions peppered throughout the stone and an SI2 diamond with a white feather inclusion off to the side. According to the scale, the SI1 would be a better choice. But in reality, while the SI2' stone inclusion is more pronounced, a prong could easily hide it. Laboratory reports help jewelers immensely. But numbers can never accurately depict a diamond's beauty like a good, hard look in person.