Minecraft wiki entry for clear, associated text: Specifies the data value of the item to be cleared. If not specified, or if -1, all items that match itemName: Item are cleared, regardless of their data. Values (bitwise AND 0xFFFF) that are invalid for the specified item id revert to 0.
Must be a 32-bit integer number. Must be between -1 and 2,147,483,647 (inclusive, without commas).