The spiritual burning of a body after a person passes away to turn their body to ash. Most people do it so they can keep their loved ones with them without having to go all Addams-Family-Carrying-Around-A-Corpse
The reason they say this is not what you think. See, the key slot in the elevator carriage is so that the firemen can use a specially issued key to control the elevator on the go and move up and down at will, allowing them to hop from floor to floor while fighting the fire from inside the elevator. They ask you not to use the elevator during a fire so that you don’t interfere with the fire brigades use of it to fight the fire.
Knowing that almost makes it funnier, imagining the firefighters being like "sorry, I know you're dying of smoke inhalation up there, but we gotta use the elevator to put out the fire up there, THEN you can evacuate."