Minecraft wiki entry for Development_versions, associated text: It is an intangible block used to indicate to the structure blocks that certain air blocks can be overridden by previously existing blocks in the destination.
Intended to be a replacement for the function barrier blocks used to convey (during early development), so they can form part of the structure itself.
They are invisible.
They are visible only when they are inside a structure being saved by a save structure blocks and the toggle option "show invisible blocks" is true.
Additionally, any placed structure void instantly becomes visible when the player holds the item in their hand, similar to the barrier.
Allows for empty spaces in a structure.
Unlike air, it does 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 becomes air but the bottom remains stone.
Version exclusive: Has a different texture than Java Edition.
Version exclusive: Has a full block hitbox.
Available through Experimental Gameplay.