DOF is a separate piece of software that you can install on your system to control feedback devices. It’s not really a full separate program; it’s more of an add-on that works together with Visual Pinball and other software. DOF handles the communications between the pinball software and your output controller devices. - DOF Setup

How VP and DOF communicate

DOF doesn’t actually talk directly to VP. It talks to VP through the B2S Backglass program, which is another VP add-on that handles the backglass artwork display.