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A structure void is used with Structure Blocks. When structure voids are saved into a structure, they will not override any blocks they are loaded into it. For example, saving an air block below a structure void will, if loaded into a patch of dirt, produce an air block below a dirt block. When saving a structure, if "Show invisible blocks" is enabled, they will appear as small red cubes.
The appearance of the structure void block is an invisible cube that is smaller than the normal block, being about half of the size of a normal block.