When the CSV file is imported on the ePO Menu, DLP Policy Manager, Ruleset, Device Rules, Exclusions, Serial Number & End User Pair page, the definition adds the @ character to the new entries. The reason is that the exported CSV file is updated with the new entries before import, and results in a conflict. 

The @ character is added to the new entries ONLY when the CSV file is imported from the Device Rules, Exclusions, Serial Number & End User Pair page.