The Section Modulus takes the Second Moment of Area (measure of the stiffness of a object due to its geometric shape) and divides this by the distance from the Neutral Axis to the outer surface of the object.
For a cross arm that is 0.1m wide by 0.15m deep the second moment of area vs horizontal is 1,250cm4 and the second moment of area vs vertical is 2,812.5cm4.
See Crossarm Parameter: Second Moment of Area for more information.
The y dimension would be given as per the below diagrams of the cross arm:
Hence:
Note that these two values above match what Neara calculates using the button Derive assuming rectangular cross section as seen here:
