A solution is not limited to source code of your projects, it also contains references to libraries that are used in your solution. It is very convenient to have access and opportunity to navigate to symbols from those libraries as if they were a part of your solution. ReSharper provides a built-in decompiler and a lot of features that help you view and navigate through compiled code.
For the second type of commands, you can specify how ReSharper will represent library symbols. When this kind of navigation is performed for the first time, you will see a dialog, suggesting to choose the default navigation action:
You can configure the default navigation on the Tools | External Sources page of ReSharper options (Alt+R, O).