I mentioned earlier an air traffic controller saying that FL180 didn't exist today, and that I would explain later. It's later now. Altimeters work based on air pressure changes with altitude, so they need a way to compensate for air pressure changes that are not associated with altitude change. So they have a calibration factor, called the QNH in Europe and the altimeter setting in North America, that you reset according to ATC instructions as you go from place to place. Once you get high enough that you are over all the mountains and short-range traffic, the benefit of adjusting for local changes in air pressure becomes less in comparison to the benefit of not having to keep changing your altimeter setting all the time, so above that altitude, the transition level, you use the standard altimeter setting everywhere.
In non-arctic Canada, the transition level is 18,000'/FL180. That means that we fly using the local altimeter setting all the way up to 18,000' and then we switch over to the standard altimeter setting of 29.92". There are plenty of airplanes flying IFR at 17000', using the local altimeter settings and if the local altimeter setting is low, an airplane flying at FL180, using 29.92 is not a thousand feet above the airplane flying at 17000', using the local altimeter setting. For every tenth of an inch below 29.92, that FL180 airplane is a hundred feet closer to the 17,000' one. But for safety there is supposed to be 1000' separation. So there's a rule.
AIM 6.4.3 Vertical Separation Between Flight Levels and Altitudes ASL
When the altimeter setting is less than 29.92” Hg, there will be less than 1 000 feet vertical separation between an aircraft flying at 17 000 feet ASL with that altimeter setting and an aircraft flying at FL180, (with altimeter set at 29.92” Hg); therefore, the lowest usable flight level will be assigned or approved in accordance with the following table:
Altimeter Setting Lowest Usable Flight Level 29.92” or higher FL180 29.91” to 28.92” FL190 28.91” to 27.92” FL200
So on low pressure days, flight level 180 doesn't exist, but you won't be assigned 18,000' either. If you had a real operational need to fly at 18,000' when FL190 was the lowest avaialble flight level, I don't know how they'd handle it, probably with a block altitude, same as if you needed to fly between 17,000' and FL180 on a high pressure day.