As far as I know there is no exact documentation for this. As the best practice, I would suggest you to keep log level to default unless Qlik Support ask your to turn it to debug level for trouble shooting purpose. Talking about log, Qlik Sense has "Centralized logging" feature as well. When the feature is enabled, logs will be written into DB apart from physically generated into disks. Unless you have particular needs, you don't need to enable it, as it may add traffic to DB as well as adding DB maintenance work for you. Logs will be generated into disks no matter "Centralized logging" feature is enabled or not.