Proof that it has been legally imported after meeting border requirements on such things as safety standards (e.g.TPMS) and paying any duty that may be levied (import duties were waived on our vehicle because it had been originally manufactured in the US). At the registration office in Vermont we had to provide proof of transfer of title, which for us was ICBC insurance registration -- they kind of choked on that because their system uses title certificates, but they worked through it with higher level supervisors after a couple of visits.