The layer structure of the default responsive css files is (from bottom to top): global → default → narrow → normal → wide
How the CSS Files Activate
Knowing this, let's look at those five CSS files again.
global.css: This CSS file always activates. In one sense, it's the "mobile" stylesheet, since it's the only CSS that will activate for a mobile site.
But global.css is also, well, global, since it activates for every width. For instance, if you set a heading font here, the headings will be that font no matter what the size.