Originally a box strength was calculated by its resistance to perforation. Back then most boxes were used for train transport. So the merchandise used to sustain a lot of lateral pressure due to the train’s movement and other items in the wagon. This let to the development of a standard called the Mullen Test. This test is “A test performed to measure the bursting strength of paper or paperboard”. It is usually calculated in either pounds per square inch or kilopascals. Now a day, we use mostly truck transportation and pallets. When a box is attached on a pallet we need to know how high we can stack them. This test is expressed in Kilonewtons per meter (kN/m). Equivalence test have been developed to pass from the Mullen to the ECT. So this is why a 200 Mullen is the same (almost) as a 32 ECT.
source: printwiki.org