Flight levels define altitudes based on a standard pressure of 1013.2 mbar = 29.92 inHg.
All aircraft operating on flight levels calibrate to this same standard setting regardless of the actual sea level pressure.
Flight levels are assigned a number, which is this nominal altitude ("pressure altitude") in feet, divided by 100.
For example, altitude of 32,000 feet is referred to as flight level 320.
An aircraft flying at flight level 320 will usually be at some other actual height above mean sea level than 32,000 feet.
Since all other aircraft set their altimeters to a standard pressure, no collision risk arises.
To avoid collisions between two planes, their real altitudes are not important, but only the difference between them.
This difference solely depends on the air pressure at both planes,
and does not require knowledge of the local air pressure on the ground. /Wikipedia/

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