Texas info (Your state may vary.): There are two types of expunctions, those of records and those of convictions. The latter are quite rare, in that proof of innocence of the original conviction is required. The former involves filing a petition for expunction, notifying all appropriate authorities, having a hearing, and getting a court order to expunge the records. That order is then sent to all agencies that might have records to be expunged, who are then required to destroy the records. This would be appropriate where charges are filed for a crime, but are later dismissed, or at the extreme, where a case went to trial and the defendant was acquitted and all appellate remedies are either exhausted or mooted by the expiration of appellate deadlines.
Here's the problem: It is virtually impossible to be certain that a) every agency with records has been notified, and b) even if they have, that they have actually destroyed the records. Since Texas requires that all criminal charges above a Class C Misdemeanor be disclosed on a CHL application, unless expunged, I tell my students that even if there's been an expunction, it is likely better to disclose, and just provide certified copies of the dismissal documents with the application. I've helped a few students obtain the appropriate documents.