What do the cleanroom grades A, B, C, and D mean?

In cleanroom modular systems for sterile room drug manufacturing, different grades are defined based on the cleanliness required. Grade A cleanrooms, equivalent to ISO 5, are used for high-risk activities like aseptic connections, while Grade B supports aseptic preparation and is ISO 5 at rest and ISO 7 in operation. Grades C and D are used for less critical stages in cleanroom projects, with Class C at rest being ISO 7, and Class D being ISO 8.