Mistaken Identity vs Mistaken Personality

I'm really stuck on this one. Can someone please explain the difference between Mistaken Identity and Mistaken Personality in Contract Law? Dono mein kya fark hai exactly? In Donoghue v Stevenson, if the plaintiff mistakenly identifies the defendant as the manufacturer, is that a Mistaken Personality or Identity? Similarly, if the defendant is indeed the manufacturer but the plaintiff doesn't know, is that a Mistaken Personality or Identity? Please help! How do you guys make out this distinction?