Rice hulls are the one entry on a malt reference list that isn't really a malt or an adjunct in the usual sense — they're the outer husk of rice grains, contributing essentially no starch, no fermentable extract, no flavour, and no meaningful colour to a beer. Their entire job is mechanical: rebuilding the loose, porous filter bed that barley's own husk normally provides during lautering, for mashes that don't have enough natural husk material of their own.
A high-wheat, high-rye, or high-oat mash runs real risk of a slow or completely stuck sparge, since wheat, rye and oats all lack barley's protective husk structure — rice hulls, added at dough-in rather than as a rescue measure once a sparge is already struggling, solve that problem directly without contributing anything else to the recipe. Because they add nothing to gravity, colour or flavour, rice hulls don't appear as a percentage-of-fermentables figure the way every other entry on a grain bill does; they're simply added in a quantity proportional to how much husk-free grain the recipe is using, typically half a pound to a pound for a 5-gallon batch pushing a wheat, rye or oat addition toward its upper range. There's no substitute for their specific mechanical role — a stuck mash from insufficient husk material has no fix except adding hulls, or in a real pinch, running a longer, slower, more patient sparge instead.