The standard pressure region is airspace that includes the Northern Domestic Airspace and all airspace above 18,000ft within the Southern Domestic Airspace. When flying in the Standard Pressure Region, your altimeter is set to 29.92 in Hg and all altitudes are expressed as “flight levels.”
FL210 = 21,000 ft
FL90 = 9,000ft
To maintain vertical separation between aircraft flying high in the Altimeter Setting Region of the Southern Domestic Airspace and the Standard Pressure Region, there are lowest useable flight levels for lower than normal pressure measurements.
If the altimeter setting is 29.92 in Hg or higher, the lowest usable flight level is FL180
For 28.92 to 29.91 in Hg, it’s FL190.
And for 2792-28.91 in Hg, it’s FL200.