What is the function of parameter connection_header in write function in rosbag?

    //! Write a message into the bag file
/*!
 * \param topic The topic name
 * \param time  Timestamp of the message
 * \param msg   The message to be added
 * \param connection_header  A connection header.
 *
 * Can throw BagIOException
 */
template<class T>
void write(std::string const& topic, ros::Time const& time, boost::shared_ptr<T const> const& msg,
           boost::shared_ptr<ros::M_string> connection_header = boost::shared_ptr<ros::M_string>());