What is UL listed or UL classified?
The UL symbol / insignia stands for: "Underwriters Laboratory". This insignia is an Insurance Industry organization, that now is independent and non-profit. The underwriters Laboratory tests electrical components and equipment for potential hazards. When something is UL-listed, that means that the UL has tested the device, and it meets their requirements for safety - ie: against fire or shock hazard. This doesn't necessarily mean that the device actually does what it's designed to do, just that it probably won't harm you electrically via shock.
The UL (Unwriters Laboratories) does not have power of law in the U.S. -- you are permitted to buy and install non-UL-listed devices. However, insurance policies sometimes have clauses in them that will limit their liability in case of a claim made in response to the failure of a non-UL-listed device. Furthermore, in many situations the NEC will require that a wiring component used for a specific purpose is UL-listed for that purpose. Also, be cautious when installing a non-approved, non-UL listed piece of equipment. Most health inspectors DO require this and will enforce standards otherwise not set by the government.
Indirectly, this means that certain parts of your wiring must be UL-listed before an inspector will approve it and/or occupancy permits issued.
So, prior to purchasing a Freezer or any other refrigeration equipment for your restaurant, convenient store, etc., it is wise to know what the local inspector will require and/or allow. Check the data plate and be a wise consumer.
Most, if not all refrigeration equipment made today already complies with these laws and are UL listed.
I do however caution against "older" equipment though as prior to these laws being passed; many, particularly deli cases are not UL listed or approved.
Where is the UL insignia traditional located and how do I know if a piece of equipment is UL approved?
The UL insignia is typically located on the "data plate" on any given piece of equipment. Traditionally this data plate is located inside the left door towards the top of the unit.
When purchasing a piece of equipment - new or used, be sure to ask your salesman if the equipment is UL listed or classified and if so; have them show you the tag.