Minecraft wiki entry for Block_entity_format, associated text: : The block entity's root tag.
 movingBlock: The main layer of moving block represented by this block entity.
Tags common to block[show]
 movingBlockExtra: The extra moving block layer represented by this block entity.
Tags common to block[show]
 pistonPosX: The X position of the piston base.
 pistonPosY: The Y position of the piston base.
 pistonPosZ: The Z position of the piston base.
 movingEntity: (May not exist) The block entity stored in this moving block.
Tags common to block entity[show]