Off-label use means any use of an FDA approved medical device or pharmaceutical for which it was not specifically approved. The off-label use of a device is considered appropriate when a surgeon in his or her opinion deems the use to be beneficial for the patient. There are many, many examples of off-label use, such as giving a person suffering from a heart attack an aspirin. Aspirin is not approved to treat heart attacks.
Excimer lasers are approved for use on virgin eyes, therefore any enhancement surgery would be considered an off-label use of the excimer laser, however this use would be within the "practice of medicine" rules and thereby appropriate.
FYI: At first all excimer lasers were approved for PRK and Lasik was an off-label use. Recent approvals are for Lasik and PRK is an off-label use.