Sys identifies important files for the MS-DOS and Windows operating systems. These files may contain settings and variables depending on their nature, but also provide essential functions (system configuration files). SYS files are mainly used for device drivers (system device drivers).
Under MS-DOS and Windows 95, files like io.sys, msdos.sys, and config.sys were very important, but are no longer relevant. SYS files belong to the driver model of Windows and are often also part of drivers from other manufacturers. In addition to the usual system folders such as Windows / system32 and Windows / winsxs (as of Vista), these can also be located in the program or manufacturer-specific folders.
Generally you can not just install SYS files. Either they are copied via a driver installer to the right place or you need the matching inf files, which can be installed via context menu (right-click). In both cases, but still some other files are included.