The National Information Exchange Model (NIEM) is an XML vocabulary that is quickly gaining popularity as a means to support cross-domain information sharing. NIEM is about the semantics of the message, not transportation of that message. “Web services” is a term for a group of industry standards that collectively provide a mechanism for exchanging XML-based messages, such as NIEM messages. One of these standards is the Web Services Description Language (WSDL), which standardizes the specification and description of a web services interface. This technical brief (see source below) explains the crucial relationship between NIEM Information Exchange Package Documentation (IEPD) and WSDL.
The information that travels between systems is critical to the workings of a web service. A WSDL has the responsibility of specifically defining what that information looks like. Though IEPD is a concept separate from web services, an IEPD can be used in conjunction with WSDL to clearly and unambiguously specify a web services interface, including all the semantics. In other words, web services can be used for exchanging messages defined by NIEM IEPDs.
For further reading, please see the source below.
Source - Using NIEM with Web Services