Good answers TJ959,
I just thought I'd jump in with the specific definitions according to FedExCC, which vary a little with each company, FedExCC is
more specific than some others.
A (Mini Van)
Cargo payload capacity of 500 pounds
48" high door opening (minimum)
48" wide door (minimum between walls)
B (Full size cargo van)
Cargo payload capacity of 2,500 pounds
Minimum 8-1/2' foot cargo bed
48" minimum width between wheel wells, door opening 48" high minimum, 48" wide
C (Straight Truck) 14' Weather-tight Cargo Box Vehicle
Unit must be able to legally scale 5,000-pound payload
Box floor must be dock high, 48-52" from the ground
Inside dimensions must be 90" wide x 92" high
Safety support legs at the rear of the frame of the unit (minimum rating 25,000 pounds)
Rear door opening must be minimum 88" high, 85" wide (roll up or swing doors are optional)
Legal sleeper between cab and box
D (Straight Truck) 22' Weather-tight Cargo Box
Unit must be able to legally scale 13,000-pound payload
Box floor must be dock high, 48-52" from the ground
Inside dimensions must be 90" wide x 92" high
Rear door opening must be minimum 88" high, 85" wide (roll up or swing doors are optional)
Safety support legs at rear of frame of unit (minimum rating 35,000 pounds)
Legal sleeper between cab and box
E (Tractor Trailor) Tandem Axle Tractor with Sleeper
Cargo payload capacity of 44,000 pounds
I know FedEx may differ a little on the definitions than some companies, but in general, these are the requirements. Some may vary on weight, for example some may not require a van to be able to carry 2500, where some may require 3000 pounds, etc...
Dreamer