Today you can specify a logging file when using unified logging.

For example:

$ java -Xlog:gc+region=trace:filename=log.txt ...

The proposal is to add an event to JFR and allow users to specify jfr as output.

For example:

$ java -Xlog:gc+region=trace:jfr -XX:StartFlightRecording ...

It might be convenient to configure the logging using JFR as well,

<event name="jdk.Log">
  <setting name="enabled">true</setting>
  <setting name="filter">all=warning</setting>
</event>

Log configuration that is set using command line or jcmd acts as settings from an additional recording when combining the final state to use. More verbose tag set, i.e. gc+region, wins over less verbose.