File used by Visio, a technical drawing program; saves a collection of shapes, connectors, or other objects that can be used within a drawing; often used by organizations for distributing logos and other standard design elements that can be included in technical drawings.

To add the shapes in a VSSX file to your installation of Visio, simply double-click the file and Visio will import the shapes into your stencils library. The VSSX format was introduced with Visio 2013. Previous versions of the software use .VSS files for stencils.