This menu item toggles the line number display on the left hand side of the window.
The line numbers are shown in a fixed area on the left hand side of the Window for each file if this item is checked. The font used is automatically selected based on the current font and is slightly smaller than the selected font for display.
The color of the line numbers may be set by going to Layout -> Themes -> Manage Themes and clicking on the Editor tab and then changing the color assigned to Ruler, Line Number Column.
To print a file with line numbers, make the appropriate selection under the Page Setup dialog (Application menu -> Print -> Print setup -> Page setup).