HP Deskjet 6540 Color Inkjet Printer - Manual (Page 117)

• Lower, then upper: prints from the 250-Sheet Plain Paper Tray Accessory.
If the accessory is empty, then the printer prints from the In tray.
• Rear tray: prints from the Auto Two-Sided Print Accessory with Small
Paper Tray. This option appears only if the accessory is attached to the
printer.
HP Deskjet 6500 series printers User's Guide
117