What is the primary defining characteristic of a burial vault?

The primary defining characteristic of a burial vault is that it provides protection for the casket. A burial vault is designed to encase the casket and protect it from the weight of the soil and the elements found underground, thereby helping to preserve the casket and its contents over time. This added layer of protection is significant for maintaining the integrity of the burial site and ensuring that the casket does not collapse or degrade quickly.

Other options do not accurately represent the defining feature of burial vaults. While some burial vaults can be made of wood, this is not a universal characteristic, as they can also be constructed from concrete, metal, or other materials. There is no legal requirement for burial vaults in all jurisdictions, making the claim about legality incorrect in many situations. Lastly, a burial vault and a coffin serve different purposes; a coffin is specifically designed to hold the body, while the vault serves as an outer container that provides additional protection for the casket within. Thus, the correct answer emphasizes the primary function of the burial vault in the context of funeral service and merchandising.