The option is used to determine appropriate distances between member cross-sections protected against torsion, i.e. to determine so-called: effective length - for a cantilever. The option is accessible after pressing one of the buttons: Upper flange or Lower flange provided in the Lateral buckling parameters field of the Member Definition - Parameters dialog box.
The dialog box presents Table 14 included in the code BS5950: 2000.
The bottom part of the dialog box contains the Intermediate restraints option. If it is selected and the 
icon is pressed, then the Internal bracings dialog box opens on the screen.