Minecraft wiki entry for Flattening, associated text: Block entity data
Tags common to all block entities[show]
 Item: The Block ID of the plant in the pot. Known valid blocks are minecraft:sapling (6), minecraft:tallgrass (31), minecraft:deadbush (32), minecraft:yellow_flower (37), minecraft:red_flower (38), minecraft:brown_mushroom (39), minecraft:red_mushroom (40), minecraft:cactus (81). Other block and item IDs may be used, but not all will render. Together with Data, this determines the item dropped by the pot when destroyed.
 Data: The data value to use in conjunction with the above Block ID. For example if Item is 6 (a sapling block), the Data is used to indicate the type of sapling.