Plainbox (and its derivatives) uses a configuration system composed of variables arranged in sections. All configuration files follow the well-known INI-style syntax. While Plainbox itself is not really using any variables, knowledge of where those can be defined is useful for working with derivative applications, such as Checkbox.
The [environment] section deserves special attention. If a job advertises usage of environment variable FOO (by using the environ: FOO declaration) and FOO is not available in the environment of the user starting plainbox, then the value is obtained from the [environment] section. This mechanism is useful for distributing both site-wide and per-user configuration for jobs.
/etc/xdg/plainbox.conf
System-wide configuration file (lowest priority).
$XDG_CONFIG_HOME/plainbox.conf
Per-user configuration (highest priority).
plainbox-check-config (1)