The description of err format expansion
The err file is a text document which contains error messages, which were automatically initialized and created by particular program. The err files can be distinguished as program loads, closes or executes. Also the data ejected from these resources is often used by software developers during the profiling or testing the apps.
As far as programs usually print to original “out” and “err” streams while they are running, a developer can select whether he desires to redirect his output to logerr file object for error output or to log.out file object for standard output. Both of the mentioned file objects can be overwritten between runs of the program in order to maintain the history over successive runs.