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'''$FG_HOME''' is a placeholder variable representing the path to where user-specific FlightGear data is stored (not application data). Unlike $FG_ROOT, $FG_HOME is NOT an environment variable that needs to be specifically configured for FlightGear to work. $FG_HOME is the only place were FlightGear data is written to, $FG_ROOT is generally read-only.
$FG_HOME is only used to refer to the OS-specific location for some settings/folders. The actual location of $FG_HOME will be determined by the fgfs binary during startup, it is a name for a directory that varies with the various operating systems supported by FG (Windows, OSX and Linux) and it stores information between sessions, such as: configuration/preferences, properties marked with the "user-archive" attribute and aircraft-specific settings (using the data helper class in aircraft.nas). $FG_HOME is also the location for the SQLite-based NavDB cache.
($FG_HOME and the '''/sim/fg-home''' property should be considered read-only, because they cannot be affected during startup or at runtime)