The first argument is the filename to which log records should be written. This is used exactly like the argument to TransferLog; that is, it is either a full path or relative to the current server root.
The format argument specifies a format for each line of the log file. The options available for the format are exactly the same as for the argument of the LogFormat directive. If the format includes any spaces (which it will do in almost all cases) it should be enclosed in double quotes.
Instead of an actual format string, you can use a format nickname defined with the LogFormat directive.