NEC classifies the 18" above the floor in repair rooms where Class 1 liquids (I assume gasoline) are transfered. What do they mean by transfered? Does it mean dispensed? Or can it mean any place a gas tank may be replaced?

I think the assumption is that fuel, oil or other flammables may be present on the floor of any commercial garage. That is not an unreasonable assumption. You don't have to drop the fuel tank to spill gasoline. Replacing a fuel pump or working on the carbs/injectors will do it. For that matter motor oil is a flammable liquid. It just has a higher flash point than gasoline. There are also a lot of flammable solvents used in a garage.

The last debate of this subject resulted in definitions something like this:
"Transfer" does not mean "Dispense" as dispense relates to the service station (pumps) now 'dispensers'.

The opinion was based on the heavier than air vapors collecting within the 18" zone.

48" might work out better for the end user once you put in benches or equipment.

I should have added that most, if not all jobs that I have been at/on, everything was above 36" minimum; 48" for devices.