Most people think of a classifier as a screen, something used to remove larger rocks before panning or sluicing, and while they are used for that, classifiers are really about sorting by size - classification.

The grizzly on your highbanker, the screen drum on a trommel, and a 100-mesh sieve all serve the same purpose - to sort material by size. The reason is straightforward. If everything is the same size, gold will be heavier than most everything else.