The following table shows all of the classes within ISO8573.1. To choose which class meets your requirements, assess what dewpoint you need, the permissible remaining levels of oil and remaining dirt particles.
The class is specified as x-y-z where x, y and z are as follows:
x = the particle size as per column 2 below
y = the pressure dewpoint as per column 3 below
z = the oil content as per column 4 below
ISO8573.1 Class 1-4-1 for example would be a refrigeration dryer with a high quality coalescing filter to remove particles to 0.1 micron, reduce the pressure dewpoint to +3°C and remove the oil content to 0.01 Mg/m³.