The option allows defining appropriate distances between sections of a member blocked against torsion - the so-called lateral buckling length. It is available after pressing one of the buttons: Upper flange or Lower flange in the Lateral buckling parameters field in the Member definition - parameters dialog box.
If the user wants to eliminate lateral buckling effects from the calculations, the 
icon has to be chosen. If pressed, lateral buckling will be disregarded in the calculation process.
The lower part of the dialog box contains the Internal bracings option. Selecting it and pressing the 
icon opens the Internal bracings dialog box.