What are the differences between Unsolicited messaging and Class data messaging on DNP?

In the DNP protocol, unsolicited messaging and class data messaging are two different ways that
the slave device will send event data (change of state data) to the master. Unsolicited
messaging is sent automatically by the slave device when a point-to-point relationship exists
between a master and one slave when they are the only two nodes on a DNP network.Class data
is requested bythe master device (polled).

Please see the appropriate DNP or DNPSNET User Manual for more information.