Having a case dismissed is only one part of the process of clearing someone’s arrest record. In Tennessee, once a case has been dismissed it is then eligible to be expunged from your record. Expunging a record removes the existence of the case altogether from your criminal history. It’s the difference between a record which reflects someone who has never been arrested, and someone who was once charged with a crime but later the charge was dismissed.
For example, if you are charged with a felony drug offense and the prosecutor decides later to dismiss the case; anyone who runs your background will see that you were charged with that crime. Had it been properly expunged, any background check would reveal nothing – they would never know you were ever charged with anything.