This is metaphysical knowledge. Underworld currency is the currency used by the world for use in the underworld. It must be burned before it can be delivered to someone who has passed away. Metal coins are one level higher than paper coins and have the meaning of Yangjin. There is a saying in the underworld that "money can make ghosts grind", so pressing coins on the four corners of the bed is also a way to pray for the blessings of ghosts and gods.