DOS was modified to search the root directory of the A and C drives in that order for a config.sys file. If config.sys is found, that drive becomes the default drive and config.sys and any autoexec.bat is processed off that drive. If neither A nor C contains a config.sys file, D becomes the default drive and the default config.sys and autoexec.bat are processed.
The Boot options were tailored for the HP palmtop to provide for overriding this default config.sys search behavior.