Minecraft wiki entry for Structure_Block_file_format, associated text: The root tag.
 DataVersion: Data version of the NBT structure.
 author: Name of the player who created this structure. Only exists for structures saved before 1.13.
 size: 3 TAG_Int describing the size of the structure.
 palette: Set of different block states used in the structure.
A block.
 Name: Block ID.
 Properties: List of block state properties, with [name] being the name of the block state property.
 Name: The block state name and its value.
 palettes: Sets of different block states used in the structure, a random palette gets selected based on coordinates. Used in vanilla by shipwrecks.
A set of different block states used in the structure.
A block.
 Name: Block ID.
 Properties: List of block state properties, with [name] being the name of the block state property.
 Name: The block state name and its value.
 blocks: List of individual blocks in the structure.
An individual block.
 state: Index of the block in the palette.
 pos: 3 TAG_Int describing the position of this block.
 nbt: NBT of the associated block entity (optional, only present if the block has one). Does not contain x, y, or z fields. See Block entity format.
 entities: List of entities in the structure.
An entity.
 pos: 3 TAG_Double describing the exact position of the entity.
 blockPos: 3 TAG_Int describing the block position of the entity.
 nbt: NBT of the entity (required). See entity format.