The ISO 4406 standard utilises a three number system to classify system cleanliness — example: 15/13/10. The first number means there are between 160 and 320 particles >4 micron, between 40 and 80 particles >6 micron and between 5 and 10 particles >14 micron
The second number represents particles greater than 6 micron and the third number represents those greater than 14 micron.