Air cleanliness class

Particulate load of the surrounding air which does not exceed a defined total maximum particle size distribution per volume for defined particle sizes when the set-up is not in operation. Classification, for example, in accordance with DIN EN-ISO 14644-1: Cleanrooms and associated cleanroom areas.
from DIN EN-ISO 14644-1

Reference Volume (and volume-specific, additional or differing definitions to the standard):
VDA volume 19