The FCSTD file format is used by FreeCAD, an open-source parametric 3D CAD modeler. It is the default file format for saving and opening projects within FreeCAD. FCSTD files contain information about the 3D models, sketches, and other design elements created within the software. These files can be easily shared and edited by other FreeCAD users, making it a convenient format for collaborative design projects. FreeCAD also supports the import and export of other file formats, such as STEP, IGES, and STL, allowing users to work with a variety of CAD software. Overall, the FCSTD file format is an integral part of the FreeCAD ecosystem, providing a versatile and reliable way to store and exchange 3D design data.