Standardization is the adoption of generally accepted uniform procedures, dimensions, materials, or parts that directly affect the design of a product or a facilit.
Workflow as any commonly used service in software application has its standards. Workflow management collation WfMC is one of organizations concerning about developing standards for workflow and business process management systems.  XPDL and Wf-XML are two standards developed by WfMC. 
XPDL is a standard workflow modeling language. XPDL is the Serialization Format for the standard Business Process Management Notation (BPMN). BPMN standard defines the look of how the process definition is displayed on the screen. XPDL provides a file format that supports every aspect of the BPMN process definition notation including graphical descriptions of the diagram, as well as executable properties used at run time. With XPDL, a product can write out a process definition, and another product can read it and reproduce the same diagram that was sent. XPDL is used today by more than 80 different products today to exchange process definitions.
The other standard developed by WfMC is Wf-XML. Wf-XML is based on XML format for process data exchange.  Wf-XML has been developed to enable workflow interoperability over the internet. Wf-XML is a standard webservice definition written in WSDL to enable process definition exchange over the internet. Using Wf-XML a workflow engine can invoke a process in another workflow engine, and wait for it to complete. Also A process design tool could use this standard web services based protocol to browse processes on remote WfMS server.

WfMC developed a WfMS model that shows how WfMS can interface with other systems.  The above figure shows this model.  It shows how standardization is important to WfMS, where WfMS has to interface with many other systems as shown in the figure. It may interface with other engines and used by other applications.