$System
is a string describing the type of computer system on which the Wolfram System is being run.
Examples
open allclose all
Wolfram Research (1988), $System, Wolfram Language function, https://reference.wolfram.com/language/ref/$System.html.
Text
Wolfram Research (1988), $System, Wolfram Language function, https://reference.wolfram.com/language/ref/$System.html.
CMS
Wolfram Language. 1988. "$System." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/$System.html.
APA
Wolfram Language. (1988). $System. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/$System.html