Many band manufacturers base their resistance level on different things. Some combine the total of both ends of the bands, some use the total of the full elongation and some literally just apply a random number. Bodylastics very accurately tests the resistance levels of their bands.
Bodylastics determines the resistance of their bands by measuring the resistance at 200% elongation (approx. 8 feet). Resistance bands provide what is called progressive resistance. The further they stretch, the more resistance they provide. For example, a single length of our 8lb red band will provide 8lbs of resistance at 200% elongation and 24 lbs of resistance at full elongation, which is 4 times it's length (approx. 16 feet).
Currently our largest band is the Bodylastics black 40lb band. Technically it is a 80lb band as it provides 40lbs per side. If you clip the end of the band to the wall and pull on a single length, the band will provide 40lbs. at 200% elongation. If you have the band anchored in the middle and have a handle on each end of the band, it is providing 40lbs. to each arm. If you clip both ends to one handle it is providing 80lbs.