Out-of-order delivery is the delivery of the packet in a different order from which they were sent.
Out-of-order delivery can be caused by packets following multiple paths through a network, or via parallel processing paths within network equipment that are not designed to ensure that packet ordering is preserved.
TCP prevents out-of-order delivery and hence users would notice that whenever TCP is enabled in the transport layer, the number of out-of-order packets would always be zero.
Typically out of order delivery packets occur when running MANET, WSN simulations with UDP in the transport layer.
A document explaining the steps to arrive at the number of out-of-order packets from the event trace is attached below