Out-of-order delivery is the delivery of 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 when ever TCP is enabled in transport layer, the number of our-of-order packets would always be zero.

Typically out of order delivery packets occur when running MANET, WSN simulations with UDP in transport layer.

A document explaining the steps to arrive at the number of out-of-order packets from the event trace is available at www.tetcos.com/File_Exchange/Help/NetSim-v10-help-out-of-order-packet-calculation.docx