In order to localize report properties such as data series names, column headers, etc., there must exist an i18n key for each property. Since you can design brand-new reports that didn't exist in the out-of-the-box design, you need a way to assign i18n keys to the new reports' properties.

This need is addressed by the Unique Key fields that are found in the various report design pages. If you do not plan to localize a report, you may leave these fields empty. But if you need to localize the report, each of these fields in the report must have a value.

For details about localizing reports, see Localizing Reports.