From the Qual code sheet hanging in my office
T1 - Trailer up to 1 ton,
T2 - Trailer up to 1 1/2,
T3 - Trailer up to 5 Ton
T4 - Trailer up to 10 ton,
T5 - Trailer up to 15 ton,
T6 - All trailers non 5th wheel,
T7 - All trailers 5th wheel,
T10 - Airstream
T11 - Roller towed
T12 Trailer forward area refueller