As @JonB already told you - the category is the tag where you can filter on.

Your advice is to detect which exactly QFile will store a log by log category?

Very strange advice in my case... This assumes to 10 various log categories for 10 subclasses of MDI app.

Is it possible to add some tag to 'log' method?

As @JonB already told you - the category is the tag where you can filter on.

If you are you using your own logging messages see QLoggingCategory Class for creating your own categories and what you can do with them.

The problem is a storing in the various files, not a creation my own category.

The target QFile is used in messageHandler only.

void myMessageHandler(QtMsgType, const QMessageLogContext &, const QString &);

You cannot change/add to those parameters. The message type & context are set for qDebug() and other inbuilt callers, you cannot use those to pass extra information.

If you are you using your own logging messages see QLoggingCategory Class for creating your own categories and what you can do with them.