$KernelID
is a unique ID number assigned to each running parallel kernel.
Examples
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Wolfram Research (2008), $KernelID, Wolfram Language function, https://reference.wolfram.com/language/ref/$KernelID.html.
Text
Wolfram Research (2008), $KernelID, Wolfram Language function, https://reference.wolfram.com/language/ref/$KernelID.html.
CMS
Wolfram Language. 2008. "$KernelID." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/ref/$KernelID.html.
APA
Wolfram Language. (2008). $KernelID. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/$KernelID.html