Tier and Bins? ULEV Was Simple

Tier2 Bin5-A / LEV II ULEV?  I am troubled by the complexity of emission standard already.  The "trouble" began when California, Massachusetts and few other states decided with higher "tougher" emission standard than the federal standards.

So, what is Tier and Bins?

The emission standard applies to light weight vehicle
Tier 1 = old standard, or the initial set of standard
Tier 2 = phased in between 2004-2009, and the current standard.

Bins = from 1 to 10, 1 = cleanest, 10=least clean

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Why Bin matters? I don't know, except it says how clean the car is.
Why people always use the scale 1 to 10? Why 1 to 5? or why not just "not good", "ok" "better"?

Zero, partial (mix/hybrid - doesn't say how clean when running combustion engine),
Ultra Low, Low, and barely meet the standard low.



Tier =