Anything you want to summarise values by needs to be a Key. You can drill up and down on Keys.
Anything you define as an Item will appear only at the lowest level of drill down and should be considered as 'Additional Information'.
So, for example, if you had a summary of a company's stock you might have a summary table of
Branch, Product Group, Product Code, Product Description.
Product Code and Description are likely to be one to one matches where it is easier for people to know what the product is by description rather than code.
So Product Description would be an Item and the other fields would be Keys.
You could leave the Description as a key but that just requires more processing and a bigger resulting file for no obvious benefit. (Unless you are dealing with records where the Product Description has changed frequently or is a manually value for a generic code and you are trying to analyse what has been entered!)
I hope this helps.