However outside of Kenya all of the various letter and number grades and standards can be a daunting and confusing. Most of it stems from the fact that most coffee roasters assume that the grading system is a grading system. In other words they assume that 'grading the coffee' is giving it a grade. Like in school, where a grade of an A is better than a grade of a B. The Kenyan coffee grading system does NOT grade on quality or relative value or merit.
The Kenyan Green Coffee Grading system uses letters to denote PRIMARILY (although not exclusively) physical characteristics regardless of quality. For instance a coffee graded an "AA" is NOT the top quality (and there is no "A" nor "AAA"), in fact it has nothing to do with quality per se. An AA coffee is simply and only a coffee that has been screen sorted to select and separate only he coffees whose screen sizes are screen 19 and 19. No screen 17 and no screen 20 coffees, ONLY screen 18-19.
This is done partly from the idea that the best way to tell quality was bean size. Screen size was quality. Quality was physical, rather than sensorial. These largish beans (AA or screen 18-19) are referred to as 'bold beans' because they are 'bolder' aka bigger. A grade of an AA does not have anything to do with the actual quality of the coffee. it simply denotes the screen sorting size range.
Similarly a coffee graded to an "AB" is simply screens 16-17 and NOT a lesser quality coffee. An AB coffee from the exact same day harvest can be much better than the AA size beans from the exact same lot. Of course it can also be worse, or just different. But basically it is simply a different screen sort.
So bottom line use the Kenyan Green Coffee Grading System as a guide to tell you the screen sorting sizes of a coffee. For the most part it will not tell you the relative quality of that coffee compared to other coffees. Although some of the other grades do imply (although don't actually state) lower quality it is best to use cupping notes, cupping scores and sometimes price to determine relative quality.