The References section has a link to an article that complements the documentation well. It says (on page 3):

Each appender can define a so-called appender threshold, a minimum level required for an oncoming message to be honored by the appender:

    $appender->threshold($ERROR);
[download]

If the level doesn't meet the appender's threshold, then it is simply ignored by this appender.

Was that what you were looking for?