Routes are officially designated as belonging to upper airspace, lower airspace or both.
In the USA, high level airways (jet routes) usually have an identifier starting with 'J', while lower airways (victor airways) start with 'V'. In Europe, upper airways identifiers are usually prefixed with 'U'.
Different countries may have different regulations concerning the vertical limit of the lower airspace.
This setting is usually left to "Both", as the cruise altitude setting is enough to automatically select the proper airways using their real vertical limit. However, if you need to exclude the lower or upper airspace for some reason, it is possible to force the auto-routing algorithm to do so.
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