A system devised by expediting companies.
Definition can vary from company to company.
Basically
B- Van or Sprinter
C small straight- typically 6 pallets /5000 lbs
D larger straight- can vary but 10 pallets/12-15,000 lbs minimum
E - semi tractor
Truck classes, an industry standard is class 4 thru class 8
Class 7 is your medium duties, M2's etc.
Class 8 is your heavier trucks. Semi's are class 8.
There are heavy straights built on Class 8 chassis.