The Duet3D filament monitors sense filament movement and so can indicate a that filament has run out, jammed or has been ground away in the extruder. This is the magnetic filament monitor version . So called because it uses a rotating magnet to very accurately detect movement.
The filament monitor consists of a PCB which senses the rotation of a diametrically magnetised magnet. That magnet is in turn rotated by the filament motion via a hobbed part. The angle is reported to the Duet
Please read over the documentation on Duet3D's site as this is a more advance feature to implement. https://duet3d.dozuki.com/Wiki/Duet3dFilamentMonitor_RotatingMagnetVersion