The Max Bending Moment vs Horizontal/Vertical is used by the Finite Element Analysis (FEA) engine when assessing if the bending moments in the cross arm have exceeded the prescribed limits.
The maximum bending moment is calculated using the formula (see article on Neutral Axis for explanation of this concept):
Example
Consider the following cross arm that has a Section Modulus vs Horizontal of 250cm4 and Section Modulus vs Vertical of 375cm4.
By pressing the button Derive from section moduli and material strength the calculation performed as follows, matching the screenshot below:
