Broadly speaking, there are three kinds of messages that the stylesheets produce: debugging messages, status messages, and errors. Debugging messages are controled by the $debug parameter. Error messages cause stylesheet processing to abort. Status messages fall somewhere in between. They alert you to facts about your document that you might want to fix, for example, broken cross references. If you ignore them, the stylesheets will produce some output.

Previously, status messages were always printed. This parameter has been introduced to provide more control. If $message-level is 0, most status messages will be suppressed. A value greater than 0 will cause some or all status messages to be printed. (At the time of this writing, all messages are at level 1, but it’s possible that more detailed messaging will be introduced in the future.)