The environmental variables used by Freeciv are documented in the ENVIRONMENT sections of …/doc/man/freeciv-client.6.in, …/doc/man/freeciv-server.6.in, etc.; or rather in the resulting man pages freeciv-client(6), freeciv-server(6), etc. for *NIX platforms.
N.B., on some platforms, where the colon ':' is used for other purposes, a semi-colon ';' separates directories in environment paths. These platforms also prefer back-slashes '\' instead of slashes '/' to separate path-components, because slashes could introduce (shell) command options. GNA#24781
FREECIV_DATA_PATH[]
FIXME, this was (or still is?) also known as FREECIV_DATADIR.
The default contains the current working directory, its data subdirectory if existing, the ~/.freeciv/2.x directory for version 2.x, and the installation directory determined by make install not expected to exist on Windows.
Many command line shells interpret a tilde ~ as shorthand for the HOME-directory, arriving at $HOME/.freeciv. On Windows ~/.freeciv boils down to either %HOME%\.freeciv if an environmental variable HOME exists, or to %APPDATA%\.freeciv otherwise.
FREECIV_PATH[]
An optional environmental variable FREECIV_PATH was supported in older versions before 3.0, and could be used to overwrite the default FREECIV_DATA_PATH. Also see …/INSTALL. Starting with Freeciv 2.3 the FREECIV_PATH was only used if no FREECIV_DATA_PATH existed; in essence it was renamed.
FREECIV_SCENARIO_PATH[]
The default contains any directories in a FREECIV_PATH (only for 2.x), the current working directory, its data/scenarios sub-sub-directory if existing, ~/.freeciv/2.x/scenarios for version 2.x, ~/.freeciv/scenarios for scenarios not necessarily depending on the version, and a TBD directory where the files are placed by make install.
FREECIV_SAVE_PATH[]
By default Freeciv looks in the current working directory and the ~/.freeciv/saves directory. Freeciv 2.x checks any saves sub-directories of directories in a FREECIV_PATH before these defaults, if no FREECIV_SAVE_PATH is defined.