Minecraft wiki entry for BE, associated text: Block entity data
Tags common to all block entities[show]
 blockState: The moving block represented by this block entity.
 Name: The namespaced ID of the block.
 Properties: Optional. The block states of the block.
 Name: The block state name and its value.
 extending: 1 or 0 (true/false) – true if the block is being pushed.
 facing: Direction that the piston pushes (0=down, 1=up, 2=north, 3=south, 4=west, 5=east).
 progress: How far the block has been moved. Starts at 0.0, and increments by 0.5 each tick. If the value is 1.0 or higher at the start of a tick (before incrementing), then the block transforms into the stored blockState. Negative values can be used to increase the time until transformation.
 source: 1 or 0 (true/false) – true if the block represents the piston head itself, false if it represents a block being pushed.