What is Cleanliness (CL/) in the HVAC system and clean-room's certification ? I think it is uncommon HVAC work ..please pay some Attention over it

Due to contamination during storage, transportation and construction phases, a lot of dust may accumulate in the ventilation ducts. After construction process, pollutants accumulate into duct surface during the operation of the air handling system. The aim of this workwas to develop a cleaning concept to maintain the cleanliness of HVAC system.