What is a SY_ file?
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.
expand [/r] <source> <destination> expand /r <source> [<destination>] expand /i <source> [<destination>] expand /d <source>.cab [/f:<files>] expand <source>.cab /f:<files> <destination>
When expanded, the SY_ files are converted to SYS file.
SY_ files are similar to EX_ and DL_ files.