How can I find the sky clearness index for ASHRAE clear sky model?

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Hello, I want to use ASHRAE clear sky model in DesignBuilder(EnergyPlus) which requires sky clearness as an input. As I understood this index is the ratio of daily global radiation on horizontal surface to daily extraterrestrial radiation and it doesn't exceed 1. However, DesignBuilder help says that this index can vary between 0-1.2. Besides, the fig. 5 in chapter 33 of ASHRAE handbook 2007 shows the clearness number for the US which varies between 0.85-1.2. In other sources, this number is often between 0.5-0.8. I don't really understand why this happens. Am I missing something? I would be grateful if you could help me.