Minecraft wiki entry for Development_versions, associated text: Used for the structure block to keep the blocks in that location.
Intended to be a replacement for the function barrier blocks used to convey, so they can form part of the structure itself.
They are invisible and have a small hitbox.
They are only visible when using the feature in (save) structure void.
Allows for empty spaces in a structure.
Unlike air, it will not overwrite blocks where it is loaded.
Example: If the player loads a 2-block-high structure with air at the top and a structure void at the bottom into a 2-block-high area of stone, the top block will become air but the bottom will stay stone.