Definitions: Listed vs Unlisted water heater

I often see water heaters installed on combustible floor materials. When I research is this is allowed I find this: Unlisted water heaters shall be installed with a
clearance of 12 in. (300 mm) on all sides and rear. Combustible
floors under unlisted water heaters shall be protected in
an approved manner.

Today’s inspection the water heater was built by Kenmore. The Kenmore installation instructions state:

“This water heater must not be installed directly on carpeting.
Carpeting must be protected by metal or wood panel beneath
the appliance extending beyond the full width and depth of the
appliance by at least 3 inches (76.2mm) in any direction, or if
the appliance is installed in an alcove or closet, the entire floor
must be covered by the panel. Failure to heed this warning may
result in a fire hazard.”

It specifically recommends a wood panel.

So my question is…How do you know if a water heater is “listed” or “unlisted” in regards to the combustible base issue?

I recommend they install a drip pan and then the water heater is no longer sitting right on a combustion-able material.

As as a drip pan is required in most installs anyway:

Though I have not seen anyone install a water heater on carpet before.

Many years ago before codes, water heaters had no listings and were not required to have them. As codes have developed and manufactures were required to have their products tested the specifications for their installations were required. Manufacturers installations must be listed on the front of the water heater and all plumbers are required to install as per guidelines. Water heaters are tested by ASME pressure and vessel codes, which list clearances and ratings for all water heaters.