Writes the value expressed as a id, which is the ID of the object described by this packet. IDs do not need to be GUIDs, but they do need to uniquely identify a single object within a CZML source and any other CZML sources loaded into the same scope. If this property is not specified, the client will automatically generate a unique one. However, this prevents later packets from referring to this object in order to add more data to it.
