In TSW it controls things like the gears... If you want to know what it does on specific trains go into options, controls, advanced controls, raildriver and then I believe configure and it will show you the controls of the selected train, change the train and hover over the range control to see what it does on different locos / MUs
The intent is that it be used for engines like the 363 and G6 which have high and low (road and shunting) gears.