Old-fashioned standard lead from a holding of three small cards (and sometimes more) eg you lead the nine from 9-6-3. The method has fallen into disfavour because of necessity you play a lower card on the second round, which makes it look as if you started with a doubleton.
During the play, it can be correct to lead top of nothing when conveying that you have no strength in the suit is high priority. Some pairs play top of nothing at trick one in supported suits as it is obvious the lead cannot be from a doubleton.