This allows you to select which portion of the screen will be printed or saved. The primary purpose of this option is performance and space. You no longer need to wait for the software to build a full image. It takes less time and less memory to print and disk space to save a partial image. As a side effect you can adjust where your image ends up on the page by selecting different extents. You may at anytime click on reset to put the extents back to the full image. FMSLogo remembers what you set them to even after you exit.
This menu option also allows you to set how many "turtle steps" are mapped to an inch on your printout. The default is 125 turtle steps per inch of paper. Since FMSLogo's workspace defaults to 1000x1000 turtle steps, that equates to about 8x8 inches (1000/125x1000/125). You may increase this number to take advantage of higher resolution printers but your image will become smaller. You can compensate for this by increasing your work space (and your drawing size).