Internally Flawless (IF) diamonds may contain external characteristics (also known as blemishes) that are extremely difficult to view at 10x magnification on the stone's surface, if at all, whereas Flawless (FL) is the best clarity of a diamond as it does not contain any inclusions or blemishes, under the scrutiny of 10x magnification.
Although the name is seemingly redundant, Very Very Slightly Included (VVS1, VVS2) diamonds earn this grading because, unlike their Flawless or Internally Flawless bretherin, they do contain inclusions that are so minute that they're extremely difficult to locate under 10x magnification used by expert gemologist to determine a stones clarity.
Like their higher graded counterparts, VS1 and VS2 diamonds also contain minute inclusions, such as clouds, crystals, or feathers, but, unlike Very Very Slightly Included diamonds, Very Slightly Inlcuded stones' inclusions, though miniscule are only just difficult to locate with 10x magnification, rather than being extreamly difficult.
Even without 10x magnification, Slightly Included diamonds (SI1, SI2, SI3) contain inclusions that are noticiable to the human eye such as clouds, knots, crystals, cavities, and feathers. However pronounced and numerous there are without magnification, thought, is what determines a diamond is either Slightly Included or fully Included.
Diamonds that are graded as Included (I1, I2, I3) contain very obvious inclusions that can usually be seen with the naked eye, and even more underneath 10x magnification. Included diamonds sometimes display poor transparency and lack of brilliance due to excessive clouds, feathers, knots, cavities and crystals large and deep enough to be confused for smudges or cracks. Inspite of these blemishes, Included diamonds are still able to sell well enough, inspite of being considered undesirable as compared to a higher clarity graded stone.