$SystemID is a short string that identifies the type of computer system on which Mathematica is being run.
Computer systems with the same $SystemID should be binary compatible, so that the same external programs and .mx files can be used.
Sometimes binary compatibility may only be complete when the same version of the operating system is used.
$SystemID is used in naming directories generated by DumpSave and mcc.
Values for $SystemID contain only alphanumeric characters and dashes.
Typical values are "Windows", "PowerMac", "Linux" and "Solaris".
See The Mathematica Book: Section 2.11.1, Section 2.12.5 and Section 2.13.5.
See also: $System, $Version, $MachineType, $OperatingSystem.