As per ICAO -
Generally LO airspace is the airspace extending up to and including FL 195 -
HI airspace is extending at and above FL 200 (often up to FL 600).
The "18,000 ft" limit is more of a US/Canada definition for airways structure.
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Historically, in ICAO airspace, an airway might be named i.e. A-1 (Amber 1) -
If located between same points. it would be known as UA-1 (Upper-Amber 1).
Aiirways are no longer named by "color" i.e. Amber, Blue, Green, Purple etc...
ICAO now uses Alpha, Bravo, Charlie + number... Too many airways...
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For the US, V "Victor airways" are up to 17,999 feet
At or above 18,000 ft, they are called J "Jet airways" (or HL "Hi-Level" in Canada)
Happy contrails