If, for example, you are trying to determine the classification of a carpentry shop, you will find fine sawdust mixed with larger particles of wood.
What classification fits an area where most of the Fire hazard is larger wood fibers (Class III) and a far lesser percentage is fine sawdust (Class II)?
A good choice for the carpentry shop would be a classification that erred on the side of caution. It's not a matter of "What is the cheapest lighting classification?" but a matter of "What is the safest classification to eliminate the risk of ignition."
The location's Insurer will also need to approve the final classification, and if there is a reasonable chance of fine sawdust accumulation, they will likely want the higher classification for that location.
Once you certify an LED fixture for Haz Loc Dust Classification, even if it is totally dust-proof for fine particles, it cannot be used in Class I Locations.
A dust-proof light fixture will prevent dust from getting into the fixture, but it can't stop gases.
Next: Who Decides on a Class? Who Has the Power?