Print on demand parts will automatically route through the production and print workflow when is print workflow option is checked in the part details section.
This setting will force the part into the production queue for printing and processing per the print specs added in the part details. If a part is a personalized item such as a business card, it will be added to the custom print on demand queue. If it is not a custom part it will be added to the print on demand queue.
Find more information on print on demand parts under Tools / Document Library .