Printers are increasingly supporting features such as magnetic card encoding, RFID chip programming, barcode generation, and other higher level functions that are inaccessible from the drawing layer.

Windows (as a particular example) has support for sending raw printer commands intermixed with GDI calls using the Escape API:

http://msdn.microsoft.com/en-us/library/ms535472(VS.85).aspx

Using that API with PASSTHROUGH allows an application to insert raw printer commands into the printer command stream.

It would be nice if QPrintEngine (or some other likely suspect) had a supported method of doing this on each platform. As it is, you are forced to give up QPainters functionality and generate a raw print job in the printers native language if you want to access the smallest of these features.