Coffees are graded by size because larger bean sizes usually produce better coffee.
In the grading process, coffee beans are sifted through screens, with each screen having specifically sized holes in them.
Screens are numbered 8 - 20, and the number is how many 64ths of an inch wide they are. E.g, a size 8 screen is 8/64 inches wide. The coffee’s size is the smallest hole that it passes through, e.g a size 18 if it passes through a size 18 screen but not a size 16.
- 20/64: very large. Unfortunately they are fragile and are an exception to the bigger is better rule
- 18 / 64: grade AA, also called large, superior or supremo depending on region
- 16/64: grade AB, also called large, Segundas or Excelso depending on region
Coffees smaller than size 16 would be used in cheap coffees.
Source: Coffee Grades: How Coffee Is Sorted By Size
Here’s more information on grading and classification systems in specific countries