The GIA considers various factors when grading the clarity of a diamond. They will magnify all diamonds by 10x and then look at the position of the inclusions as well as the number of them, their size, reflectivity, and colour.
These diamonds are exceptionally rare, in fact, only around one in every 5,000 diamonds have a flawless grade. This means that there are no blemishes or inclusions, even under magnification.
Internally flawless (IF)
IF are extremely similar to FL, there are only a few tiny differences, which can only be viewed by a skilled grader under magnification.
Very VERY slightly included (VVS1 and VVS2)
Diamond has small inclusions only visible under 10x magnification by a trained gemologist. No imperfections can be identified by the naked eye. A VVS2 diamond will have maybe 1 or 2 more inclusions than a VVS1 diamond. The size of the inclusions is roughly the same; often, it’s only the number of them.
Very slightly included (VS1 and VS2)
Again, inclusions are not visible to the eye. They are clearly visible under magnification but they are minor.