The core of QuakeForge's configuration is the cvar.

Cvar values.

Depending on the engine's use of the cvar, the value will be treated as a string, a floating point value, an integer value or even a vector value.

If a space is needed in the value, the value must be "quoted".

Cvar types.

From the user's perspective, there are three types of cvar:

plain cvar: the value can be displayed or set, but will not be automatically saved.
archive cvar: like a plain cvar, the value can be displayed or set, but the cvar will be automatically saved to config.cfg by the engine on shutdown or gamedir change (QuakeWorld).
read-only cvar: the value can be displayed but not changed in any way (value or flags). If the cvar also happens to be an archive cvar (the archive flag was set before the read-only flag), then the cvar will be saved to config.cfg.

Cvar related commands.

<cvar> [value]
Display the current value of the specified cvar. If value is given, set the cvar to value. Does not create a new cvar.

set <cvar> <value>
Set the specified cvar to the specified value. If the cvar does not already exist, a new cvar will be created.

seta <cvar> <value>
Set the specified cvar to the specified value. If the cvar does not already exist, a new cvar will be created. Sets the cvar's archive flag so it will be automatically saved to config.cfg by the clients.

setrom <cvar> <value>
Set the specified cvar to the specified value. If the cvar does not already exist, a new cvar will be created. Sets the cvar's read-only flag so it can no longer be modified in any way (not even by reset or resetall).

toggle <cvar>
Treat the cvar as a boolean value, changing it from off to on, or on to off. Any non-zero integer value is considered to be on, while zero and any string that parses as zero (does not start with a non-zero number) will be treated as off. The new value will always be either one (1) or zero (0).

Note: values smaller than 1 will be treated as off.

cvarlist [foo]
Print a list of all cvars known to the engine (engine created or user created). If the foo parameter is given (any value), then extra information (cvar description, flags) will also be printed.

cycle <cvar> <value list>
Cause the cvar to cycle through the list of values. Each time this command is executed, the cvar will change to the value immediately after the cvar's current value in the list, cycling back to the beginning when the current value is the last value in the list. If the current value is not in the list, the cvar will be set to the first value in the list.

inc <cvar> [amount]
Add one (1) to the cvar's current numeric value. If the optional amount is given, add that value to the cvar's current numeric value. Using -1 (inc <cvar> -1), this can be used as a dec command.

reset <cvar>
Reset the specified cvar to its default (engine specified) value.

resetall
Reset all cvars to their default (engine specified values).

Read-only cvars

Many cvars in QuakeForge are read-only because changing them at runtime would either have little meaning or be difficult to implement. However, there is a way to change even a read-only cvars: using the set command, the cvar can be created with the desired value before the engine creates the cvar (and sets its read-only flag). There are exactly three places where the cvar can be created before the engine does:

the command line (eg nq-x11 +set snd_rate 48000)
the global configuration file specified by the fs_globalcfg cvar.
the user configuration file specified by the fs_usercfg cvar.

fs_globalcfg defaults to /etc/quakeforge.conf on Linux and other UNIX like systems, and ~/quakeforge.conf on Windows (QuakeForge will expand ~ to the value of the HOME environment variable if it is set, or WINDOWS if not).

fs_usercfg defaults to either ~/.quakeforgerc or ~/.config/quakeforge/quakeforge.conf on Linux and other UNIX like systems, and ~/quakeforgerc on Windows.

The global and user configuration files are normal Quake scripts, but only set, seta, and setrom commands are executed.

It might seem strange to have the global and user configuration files specified by cvars, but QuakeForge's startup sequence is quite intense:

Execute any set commands given on the Command Line. This way, fs_globalcfg can be set.
Execute any set commands in the global configuration file. This way, fs_usercfg can be set.
Re-execute any set commands given on the command line. Thus it is possible to override fs_usercfg if it is set by the global configuration file (unless setrom is used: BOFH).
Execute any set commands in the user configuration file. Any cvars set in the user configuration file override those set in the global configuration file.
Once again, re-execute any set commands given on the command line. Thus cvars set on the command line override those set in either the user configuration file or the global configuration file (or both).

During the above process, the only cvars created by the engine are fs_globalcfg (just before reading the global configuration file) and fs_usercfg (just before reading the user configuration file). Thus, it is possible to set any cvar in QuakeForge.

The above means that:

The Command Line can be used to set any cvar in QuakeForge.
The global config file can be used to set any cvar but fs_globalcfg.
The user config file can be used to set any cvar but fs_globalcfg or fs_usercfg.
The user config file can be used to override settings made in the global config file, unless those settings have been made read-only by the global config file (by using setrom instead of set).
The command line can be used to override settings made in either the user config file or the global config file. If setrom is used on the command line, even setrom in the config files can be overridden.