Hello,
It's rather a general question about kde. Suppose I want to save some kind of information for future work with the user. I can save it in some file at his home directory or in the /etc folder. Can someone please explain me what is the rule for that, how you approach it and other tips/hints...
Depends on the kind of application, and what kind of settings, But usually, you should definitely save it somewhere in their user's home directory. That way every user keeps their own settings.