The log4j log file contains only log4j output -- and the logging therein is thus consistently formatted as per the log4j configuration.
The other log file contains the log4j output (with a few exceptions to reduce verbosity) *and* legacy output to System.out or System.err.
In 10.1 and higher these are unified into 1 log file with System.out and System.err output being routed to specific log4j loggers.
