Maintain VFR while IFR

Class A airspace starts at 18,000 MSL, not FL180 -- see 14 CFR 71.33(a). With an altimeter setting of less than 29.42, you could be legally cruising VFR at 17,500 MSL (the highest legal VFR cruising altitude) while operating above FL180. That's why the minimum usable flight level changes with lower-than-standard altimeter settings -- see 14 CFR 91.121(b) for details.

With an altimeter setting of 29.92 or higher FL180 is in Class A airspace. If the altimeter setting was less than 29.92 there should have been no mention of FL180 as it is not usable.
 
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