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We just got a postcard today from Moz saying the following:

You may have heard that custom reports aren't available in our new software platform, Moz Analytics, yet. We're working on making them easier to use, with more flexible reporting options--and more beautiful, with cool visualizations of your data.

In the meantime, custom reports will be hanging out in PRO until they're available--and awesome--in Moz Analytics.

We know it's inconvenient not to have this feature available in your Moz Analytics workflow, and we're here for you. Just give us a shout on Twitter @Moz, and we're happy to chat!

So, it seems that you have to click the "Back to PRO" button to use your custom reports, for now.

My understanding is that in the Keywords Difficulty tool, "Page Linking Root Domains" is the number of different root domains linking to that exact page that is ranking for the keyword that you entered into the tool, whereas "Root Domain Linking Root Domains" (kind of mouthful) is the number of different root domains linking to anywhere on your root domain (in other words, any page on your site). The latter should always be higher than the former, whether it's the home page or not, unless the only links you have are to your home page.