External like in a different file. This way you can use one single CSS file to style your whole site.
When you create a new CSS rule in DW you have the option to define it for the document or to add it to a new external document.
You can use more than one external CSS and some local rules. Your external CSS is loaded only one time by your browser, so your navigation is faster. AND you can create different CSS for mobile devices and print, but leave that for latter.
Once you have linked an external CSS you can move your CSS rules for your document to your external files using drag and drop in the CSS Panel.
I hope this helps
