YSlow - What does clicking add CDN do?

I was going over my YSlow results for an internal web application and I clicked the Add CDN button. Now I have an A rating for using a CDN.

What just happened?

Solution

Take a look at the YSlow FAQ on How do I add my CDN hostname to YSlow for Rule 2?

Set-Cookie and Expires headers differ
How to preload an ENTIRE web page
Avoid URL redirects
Use cookie-free domains in Opencart - YSlow recommendation
Expires and CDNS - YSlow Problems
Add Expires headers
Testing using gtmetrix.com in YSLOW shows far-future expiration error
How to enable YSlow in Firefox?
I started using a static. subdomain but YSlow still says I am not cookie-free?
Why YSlow is not detecting my cookie-free domains?
Is it really possible to achieve grade "A" in yahoo's yslow for all things for a dynamic/CMS website?
YSlow gives F grade to files compressed with mod_deflate
Gtmetrix : Add Expires headers on my .htaccess, Apache Server ,, here are the results showing the issue and my .htaccess file configuration
How to improve YSlow site performance grade for "Avoid CSS expressions"
How to solve the serve static content from a cookieless domain in prestashop 1.5?
Angular2 Implementing Performance Suggestion By YSlow
.htaccess, YSlow, and "Use cookie-free domains"
eTags misconfigured
Avoid CSS expressions
Why is my wordpress website so slow and am I having so much downtime?
tools.gzip appears not to compress content in cherrypy
How do I make my asp.net website "more cookie free"?
Which version of PhantomJS is compatible with yslow?
Gzip Compression not working
Why does google pagespeed asks to specify ETag even when cache headers are set
How to get grade A on these Yslow rules?
Would it be wise to serve static content on a cookie-free domain in SSL?
apache compression Deflate .js and .css files not compressed?
Wordpress site painfully slow
YSlow - What does clicking add CDN do?