Recently, I started using Rational Software Modeller for UML modelling. The UML diagram files that the tool creates have a *.emx extension. I was expecting it to be some proprietary binary format, but when I opened the file in Notepad, I was suprised to see a XML format!!!. It looks like even IBM has decided to make its UML models XMI compatible.So what exactly is XMI?
The XML Metadata Interchange (XMI) is an OMG standard for exchanging metadata information via Extensible Markup Language (XML). It can be used for any metadata whose metamodel can be expressed in Meta-Object Facility (MOF). The most common use of XMI is as an interchange format for UML models, although it can also be used for serialization of models of other languages (metamodels).
Here's an excerpt from Wikipedia : http://en.wikipedia.org/wiki/XMI
XMI can be used to transfer UML diagrams between various modeling tools.At the moment there are severe incompatibilities between different modeling tool vendor implementations of XMI, even between interchange of abstract model data. The usage of Diagram Interchange is almost nonexistent. Unfortunately this means exchanging files between UML modeling tools using XMI is rarely possible.