ISO classes 0 and lower do not officially exist in ISO 14644-1, but are defined by ASML based on application requirement. As an example, ISO 14644-1 class -3 must be read as 4 decades cleaner than class 1 (which is the cleanest class as described in ISO 14644-1). In Table 2, the numbers that are extrapolated from ISO 14644-1 are indicated in italic.