Range brushing an axis of the PC plot consists of selecting that axis by clicking on its name while pressing ALT, and then either choosing the Axis-->Range Brush option or pressing "r" (sometimes you may have to press "r" twice). Two to eight brushes (colors) will be applied depending on the setting in the Options-->Plot Settings menu.
When no lines are selected, this clears all existing brushes. Then all lines are re-brushed in a spectrum from the bottom of the axis to the top of the axis. This is a very effective way to quickly show structural trends in the data, particularly when there is significant overplotting.
However, if any lines are selected when range brushing happens, only the selected lines will be range brushed. They are brushed in a spectrum in a range from the lowest selected line on that axis to the topmost selected line. All other lines are unbrushed. This allows the range brushing to be dependent on another variable or a Boolean combination of other variables.
For example, in the standard selection process you might first select all lines that pass through a given set or range of values on the Variable A axis, or perhaps those that pass through those values but not other sets of values on the Variables B and Variable C axes. Then you can select the Variable D axis and range brush it, and you range brush just those lines you are interested in, with the spectrum ranging from their minimum value to their maximum value of Variable D.