D5.D.10.3 Clause 6.3.2.3(1) – LTB for rolled sections or equivalent welded section
The NBNNA recommends the use of the values specified in EN 199311 for the LTB factors λ_{LT0} and β. However it gives two different sets of values for λ_{LT0} and β based on two different conditions as give below:

If M_{cr} is determined by considering the properties of the gross cross section and the lateral restraints, the following values are used:
λ_{LT0} =0.2 and β = 1.0

If M_{cr} is determined by ignoring the lateral restraints, the following values are used:
λ_{LT0} =0.4 and β = 0.75
The program evaluates which factors to use based on the CMN parameter. If CMN = 1.0 (default) or = 0.7, then the program assumes the restraints are ignored and the second set of values is used for λ_{LT0} and β. If CMN = 0.5, then the first set of λ_{LT0} and β values is used.
These factors are then applied to equation 6.57 of NBNEN to evaluate the Lateral Torsional Buckling reduction factor χ_{LT}.