Extended Log Format
Extended Log Format (ELF) is a standardised text file format, like Common Log Format (CLF), that is used by web servers when generating log files, but ELF files provide more information and flexibility.
...
Wikipedia
Extended Log Format (ELF) is a standardised text file format, like Common Log Format (CLF), that is used by web servers when generating log files, but ELF files provide more information and flexibility.