Board Thread:General Coding Help/@comment-26424077-20160403131042/@comment-24757753-20160403132816

The 'u' stands for URL (source: Help:Including additional CSS and JS/technical).

'w' prefix (usually in the form 'w:c:dev:Script.js') is an interwiki link and uses the wiki's internal name, as opposed to the 'u' prefix which uses the wiki's URL.

Hope this helps!