A SY_ file is a compressed .sys file that is used by software installers to reduce the size of the installation files packaged inside the installer. This reduces the overall size of the installer. SY_ files can be expanded using the Microsoft Expand command line utility that expands one or more compressed files.
SY_ File Format - More Information
SY_ files are saved to disc as compressed binary files. However, their internal file format is not available publicly. The Microsoft Expand command line utility can expand SY_ files using one of the following command lines.