Visual Studio: Difference between "Save all changes" and "Save Changes To Open Documents Only"?

I'm trying to make sense of the Visual Studio options: What's the difference b开发者_C百科etween "Save all changes" and "Save Changes To Open Documents Only"?

How could you have unsaved changes in a document while it is NOT open in the IDE?

This is probably related to project files, solution files, and other files which are automatically changed without having to do any editing in a file.

If those files are under source control the effect of what is saved is much more obvious.