XSI files are used with Autodesk Softimage, an application filmmakers, video game designers, and advertising firms use to create 3D content. Although the use of Softimage has declined and Autodesk plans to deprecate Softimage in 2016, you may need an XSI file if you still use Softimage in your workflows.

To export an XSI file from SketchUp Pro, follow these steps for your current operating system:

Select File > Export > 3D Model. The Export Model dialog box appears.
Navigate to the location where you want to save your XSI file.
In the Export Type drop-down list, select XSI File (*.xsi).
(Optional) Rename your file in the File Name text box.
(Optional) Click the Options button. The XSI Export Options dialog box appears. Select your desired liveprivates options, which are explained in the following list, and click OK to return to the Export Model dialog box.
Click Export and your XSI file is saved in the location you selected.

When you export an XSI file from SketchUp Pro, you can choose from the following options:

Export Only Current Selection: If you make a selection in your model before you begin the export process, selecting this box exports only the selected geometry. When this option is deselected, the whole model is exported.
Triangulate All Faces: Select this option to break the output into triangles instead of multisided cameraboys faces.
Export Two-Sided Faces: When you select this option, faces are exported twice: once for the front and once for the back. Although this doubles the number of polygons in the exported file, the model will look more like it appears in SketchUp. Both faces will render, and materials applied to the front and back faces are preserved.
Export Edges: When you select this checkbox, SketchUp line entities are exported as XSI line entities. If you leave this checkbox deselected, edges are ignored. Usually, this checkbox is deselected because most applications ignore edges when importing XSI files.
Export Texture Maps: Select this checkbox to export textures applied to faces with your XSI file.
Swap YZ Coordinates (Y Is Up): This option does just what it says: swaps the Y (green) axis and the Z (blue) axis so that the Y axis points up. For some applications, this orientation is the default. To leave the default orientation (Z is up), leave this checkbox deselected.
Units: Select an option from the drop-down list to set the unit size in the XSI file. If you want the units to be the same as those in your SketchUp model, you can leave Model Units (the default option) selected.