The .XSX file format is used by the 'XML Schema Designer' component of Visual Studio 2005 (to access this tool, go to Project > Add New Item > XML schema).

The .XSX file holds the layout information for components on the designer surface (the .xsd file).

The XML Schema Designer is a collection of tools for working with XML schemas within Visual Studio, an Integrated Development Environment (IDE) from Microsoft for developing applications for Windows or websites.