WAS processes contain two output streams that are accessible to native code running in the process. These streams are the stdout and stderr streams. Native code, including the JVM, may write data to these process streams. In addition, the JVM provided System.out and System.err streams can be configured to write their data to these streams also.
By default, the stdout and stderr streams are redirected to log files at appserver startup, which contain text written to the stdout and stderr streams by native modules (.dlls, .exes, UNIX libraries, and other modules). By default, these files are stored as $WAS_INSTALL/logs/applicationServerName/native_stderr.log and native_stdout.log.
This is a change from previous versions of WAS, which by default had one log file for both JVM standard output and native standard output, and one log file for both JVM standard error and native error output.
See AlsoProcess logs settings