This may answer some of this.
USDOT
Registration Guide by States
You'll see it is different for different states.
Guidelines for Indiana US DOT Number and Marking Requirements
Who Must Have a US DOT Number?
If you operate a commercial motor vehicle transporting property or passengers interstate and/or intrastate and
have a GVWR or gross combined vehicle weight of 10,001 lbs. or more; you use the vehicle to transport more
than 9 passengers (including the driver); or you use the vehicle to transport placardable amounts of hazardous
materials, you must mark your vehicle with a US DOT Number. You will receive evidence of your registration
via the registration receipt.
Exemption: If you operate a farm-plated vehicle and do not leave Indiana, you do not need a US DOT number.
Commercial of course means for hire or business. Personal vehicles RV's etc are also exempt.
I figured out why most of the confusion in Indiana anyway. The law is required through the "Indiana Department of Revenue" not DOT. The DOT and police are supposed to enforce it. Violation can result in a $5,000 Civil Penalty not a motor vehicle violation.
It looks like the 10K and up is a federal law concerning travel from state to state. Some states decided to adopt it as a state law for inside their state some chose not to.
What a mess.
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