CustomLog bytes

I can't find in Apache documentation what is this for at httpd vhost template

CustomLog /usr/local/apache/domlogs/%domain%.bytes bytes

There is not such thing like "bytes".

This create file with bytes strings. WHat the sens is ?

I think they intend to add the feature of logging bytes in/out for individual domains for http/https at some point. This does require mod_logio which isn't enabled by default right now. At the moment it just fills a log file with the word "bytes." It's been the better part of a year they should either complete the feature or comment out the CustomLog. No sense in making garbage files that grow.