There are several classification systems for Cleanrooms, but the two most popular options are US FED STD 209E and ISO 14644-1. Both of these systems measure the number of particles in a cubic foot of air and assign a rating based on that measurement.
In order to meet Class 1000 / ISO 6 standards, a Cleanroom must have a maximum of 1,000 particles (sized at 0.5 µm or greater) per cubic foot of space. The number after the "class" rating indicates the contamination tolerance of the Cleanroom; for instance, Class 100 / ISO 5 standards allow for a maximum of 100 particles. Therefore, a smaller number indicates a better Cleanroom.