Printing by Source means you choose a specific tray from which you want the printer to pull paper. The printer tries to print from this tray, no matter what type or size is loaded in it. If you choose a tray that is configured for a type or size that does not match your print job, the printer does not print automatically. Instead, it waits for you to load the tray you chose with the type or size of paper for your print job. If you load the tray correctly, the printer begins printing. If you press , the display allows you to choose other trays.
HP 3550 3700 Source