If you include the control directive <ref sconf>userlog</r> in one of the configuration files then whenever the command <ref clog>Zonk log</r> is encountered when processing a template, logging information will be written to the file you specify.
The first five items shown (all starting with '$') are system information which is always generated. Any further items (only one is shown here, but you can have as many as you like) are user variables and their values. These will have been specified in the <ref clog>zonk log</r> command.
The format of the logfile is such that, if a <ref dlay>$readlog</r> command is included in a datafile, these entries will be read as variable definitions which can then be accessed just like any other variable. The '$' lines will create variables with the names as shown including the '$'.
It is my intention at some time in the future, to produce either an updated frontend or a seperate application which can be used to filter and sort log files to help with index page generation. In the meantime though, there is nothing to stop you doing this by hand if you need to.
Do not confuse a user log with the <ref sconf>system log</r> which is for debugging purposes.