Standard pallet rack has a column at every bay opening, which is a problem the moment something you store is longer than the bay itself. Cantilever racking solves that by moving the columns to the back: a single upright tower supports arms that extend out over the aisle with nothing in front of them, so lumber, pipe, conduit, sheet stock, and furniture load straight across the arms without hitting a frame.
Arms are set at the heights your material needs and adjust up or down as your mix changes, the same way beam levels adjust on selective rack. Towers are engineered to your longest piece, your heaviest load per arm, and your building height, single or double-sided depending on whether the run sits against a wall or out in the floor.