Documentation of "Key" optional Keyword

The word Key is getting overloaded here, but per my understanding of the documentation, the title is correct. There are Keywords (value, filldown, etc) and one of them is "Key". In the wiki it states:

> "Key Declares that the fields contents contribute to the unique identifier for a row."

The one test that include "Key" shows a syntax in clitable also not documented in the "CLI Table" Wiki as well. Does this only work if using multiple templates as in that one test?

Anyway to get a quick explanation of how to use this?