Those who operate in commercial commerce any vehicles meeting any of the following:
1) has two axles and a gross vehicle weight or registered gross vehicle weight exceeding 26,000 pounds (11,797 kilograms)
or
2) has three or more axles regardless of weight
or
3) is used in combination, when the weight of such combination exceeds 26,000 pounds (11,797 kilograms) gross vehicle or registered gross vehicle weight.
and
you operate in the 48 lower United States and/or the 10 Canadian provinces crossing border lines.
IF you DO NOT operate outside of your own registered state (called intrastate and not interstate) then IFTA does NOT apply to you.  However, your state probably HAS a fuel or mileage tax for you not connected to IFTA, so make sure to check with your state business tax office to be in compliance with their “inside state” rules and filing of such things!
If you do not actually need to register or pay IFTA for any of the above reasons, then this site does not apply to you.