They typically use multiple ranks of deep reeds (8', 16', and 32') to provide the "foundation" sound for an orchestra, similar to a double bass or tuba.
These often lack the standard left-hand button board. Instead, they have only a right-hand keyboard (piano or button) that produces very deep pitches, often ranging down to a pedal C . Bass Accordions
5 Different Accordion Bass Systems | All About The Accordion Bass Buttons They typically use multiple ranks of deep reeds