When background printing is turned on for printers in the Chooser, files waiting to be printed are temporarily stored in the PrintMonitor Documents folder until the printer is ready to print them. This is all done in the background.
Direct-connect printers that use Grayshare, such as the Apple Color Printer, the StyleWriter II, the Personal LaserWriter 300, also use the PrintMonitor documents folder. The host computer handles the printer spooling, and temporarily stores files in the PrintMonitor Documents folder.
The ImageWriter does not support background printing.