A bunkie board is the foundation for a bunk bed or a captain's bed mattress. It serves the same function as a boxspring on a standard bed. It is a platform that fits within the bed frame to support the mattress. Most bunk beds and captain's beds require bunkie boards to support the mattress.
The bunkie board is optional because some customers may already own one from a previous bed. It is not necessary to purchase a new bunkie board with every new bed.
These bed styles do not generally require the use of a bunkie board. If you are planning to add a trundle bed or under the bed storage drawers, you will need a bunkie board instead of a boxspring.