What does SingleKey refer to when building address locator?

[JotHall1]

While building address locators to eventually use in a composite locator I keep seeing the term SingleKey.  Can someone explain to me what it actually is?  Does it just refer to the attribute field the locator was built off?  If not, should I replace the SingleKey field with the actual field(s) I want to use for input mapping and input address fields? Any clarification would be help as I cannot find any ESRI reference material describing SingleKey. 

Thanks!

Jot 

[Ranga_Tolapi]

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