Questions about files under $APPLCSF/log

Hi

In my R12.1.3 environment,Under $APPLCSF/log,Except the concurrent manager log files,

I also found the following files

-------------

aferror.log

tempfile.bat

sqlnet.log

reviver.sh_2013-01-29_22:22:11.log

------------

In some environment,the files above doesn't exist.

Could you tell me How does these log files be generated?

why they are not exist in other environment?

what are they used for?

Thanks&Regards