What is the is_keyed Property for?

I am trying to make sure the Part ItemType is correct (our database is a bit messed up).  I have been going through and reviewing the information provided in the SolutionImportAML files and found a difference between the item_number property.  My system has <is_keyed>0</is_keyed>, but the file has <is_keyed>1</is_keyed>.  I want to understand what this is for before I change it.  Thank you.