HiPot

Shortened form of "high potential," a testing procedure using specialized equipment to determine isolation between the grounding and current-carrying conductors of a particular piece of equipment. Gear that passes will often have a "HiPot Test: Passed" or "HiPot OK" sticker.

For more, see Hipot Testing FAQ | Cirris Systems Corp.

Depending on the standard under which they are listed, Hipot testing is normally required on 100% of UL Listed products before they leave the factory.
A separate "Hipot Test: Passed" label would not be required in this case--when testing is required under the Listing, the UL Followup Service (FUS) insures that a manufacturer is performing all the tests required under the listing, including Hipot.