How to associate a configuration to a workspace

I created a C/C++ Intellisense configuration (include paths, compiler path, etc.) under a non-default name. But documentation does not explain how to use this configuration.

If I modify the default config ("Linux" in my case) with the same settings, configuration works fine (included files are parsed for example). But with the same settings in a custom configuration I can't get the included files to be found and parsed (for example). I must be missing something obvious but the documentation is not clear as to how to use (apply, associate with a workspace, whatever you want to call it) a custom configuration.