I think that rarely changing settings can safely go to app.config. It was pretty transparent when they appeared for new developers on the team, and yet you can change the settings without recompiling.
For things that change frequently and regularly, I will use either a database, a settings file in the user profile, or another shared location, etc. Something that will make it easier to manage configuration changes.
On the other hand, I do not think that the resource file should be used to store the configuration, but rather to centralize (and in some cases save translations, etc.) all hard-coded materials in your application. I would be very surprised if I inherited your code.
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