Subset 
Subset[x,y,…]
displays as .
Examples
Wolfram Research (1996), Subset, Wolfram Language function, https://reference.wolfram.com/language/ref/Subset.html.
Text
Wolfram Research (1996), Subset, Wolfram Language function, https://reference.wolfram.com/language/ref/Subset.html.
CMS
Wolfram Language. 1996. "Subset." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/Subset.html.
APA
Wolfram Language. (1996). Subset. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/Subset.html