APT_CONFIG_FILE is the file using which DataStage determines the configuration file (one can have many configuration files for a project) to be used. In fact, this is what is generally used in production. However, if this environment variable is not defined then how DataStage determines which file to use? 1)If the APT_CONFIG_FILE environment variable is not defined then DataStage look for default configuration file (config.apt) in following path: 1)Current working directory. 2)INSTALL_DIR/etc, where INSTALL_DIR ($APT_ORCHHOME) is the top level directory of DataStage installation.