XF86Config(5) manual page: "XF86Config - Configuration File for XFree86
XFree86 supports several mechanisms for supplying/obtaining configuration and run-time parameters: command line options, environment variables, the XF86Config configuration file, auto-detection, and fallback defaults. When the same information is supplied in more than one way, the highest precedence mechanism is used. The list of mechanisms is ordered from highest precedence to lowest. Note that not all parameters can be supplied via all methods. The available command line options and environment variables (and some defaults) are described in the Xserver(1) and XFree86(1) manual pages. Most configuration file parameters, with their defaults, are described below. Driver and module specific configuration parameters are described in the relevant driver or module manual page."