WHERE clause in index awareness key definition

On the "Keys" tab of a business layer object definition, there are five columns: "Key Type," "SELECT," "WHERE," "Data Type," and "Active."

Four of the five columns are fairly self-explanatory once you understand index awareness, but I'm confused about the "WHERE" column.  When is it used, and what is its purpose? 

I've tried adding it to an object to understand its behavior, but so far it doesn't seem to have any effect.

For info, I'm working with a monosource universe in IDT on BI 4.1.