In many ways, the Device interface is the heart of MuPDF.
When any given document handler is told to run the page (fz_run_page) the appropriate document interpreter serialises the page contents as a series of graphical operations, and calls the device interface to perform these operations.
Many different implementations of the device interface exist within MuPDF. The most obvious one is the Draw device. When this is called, it renders the graphical objects in turn into a Pixmap.
Alternatively we have the Structured Text device that captures the text output and forms it into an easily processable structure (for searching, or text extraction).
Some devices, such as the SVG Output device, repackage the graphical objects into a different format. The end product of these devices is a new document with (as much as possible) the same overall appearance as the initial page.
Finally, devices such as the Display List device manage to be both implementers of the interface, and callers of it. Callers can run page contents to the Display List device just once, and then replay it quickly many times over to other devices; ideal for rendering pages in bands, or repeatedly redrawing as a viewer pans and zooms around a document.
By implementing new devices callers can tap the power of MuPDF in new and interesting ways, perhaps to harness specific hardware facilities of a device.