A unit of data exchanged during an operation is called a message. You specify a message using the message element. Each message can contain one or more input or output parameters, which are defined in the part child element.

In the syntax of the message element and the part child element, MsgName is the name of the message, PartName is the name of the part, and DataType is the type of data.

In the code example, getBookRequest is the message name. It contains a string parameter called Title. The getBookResponse is the name of the second message. It contains a parameter called return, which has the string data type.