Qualified motor vehicles used/designed to transport people or property may require IFTA registration if they:
Have three or more axles
Have two axles and a gross vehicle or registered gross vehicle weight of more than 26,000 pounds or 11,797 kilograms
Are used in a combination that has a combined or registered gross vehicle weight of more than 26,000 pounds or 11,797 kilograms.

Just to confuse you… There are vehicles that fall under this definition but do not require an IFTA licence, like an intrastate vehicle.
To be licensed for tax reporting, these vehicles must use diesel, propane, or natural gas. Some jurisdictions even require gas-powered vehicles to be licensed.
Documents and applications — IFTA requirements
All carriers must keep fuel use records. Either a digital or hard copy will do. An annual license is required for every vehicle that qualifies. Each year, your base jurisdiction may send you a reminder to file your IFTA. But the responsibility falls to each carrier to remember to apply and to store the license properly once they get it.

Vehicles with active licenses are given two decals to display. Each carrier registers a base state that their vehicles operate out of. But the single license and one set of decals allows operation through all member jurisdictions. Only the base jurisdiction performs audits.

You can helpfully e-document your yearly Heavy Vehicle Use Tax Form IFTA with the IRS for just $15 through a fuel tax return. Accept your Stamped Schedule 1 is not more than minutes of e-recording.