Cutoff wavelength in an optical fiber is defined as the wavelength above which all but the most strongly guided mode are lost from the core. This occurs as a result of the way the light interacts with the structure of the fiber at a macroscopic level. The main mechanism by which these weakly guided modes are lost from the core is 'bending'. Even with the fiber in its straightest condition, macro-bending conditions still exist, so the weakly guided modes are subject to bend induced attenuation.
The bending loss of all guided modes is wavelength dependant, so this loss is tested over a spectral range large enough to capture the point at which only the most strongly guided mode remains with the fiber held in its nominally straight condition.
The 'Production/Lab/R&D' range is suitable for manufacturers of fiber and cables. It is considered unlikely that this measurement would be required in a field environment.