When Moz provides an on-page grade report card, what tells Moz which keyword to grade for?

[AliciaMarie]

I just started working with the on-page grader and of couse am particularly interested in fixing my pages with not so stellar grades. Can someone tell me how SEO Moz decides what keyword to grade a page for? I receive a report each week that tells me how many pages I have with A's, B's etc. But what tells Moz to grade a page for a particular keyword? Is it the first keyword listed in the meta description?

[KeriMorgret]

They're automatically chosen. If the keyword is ranking in the top 50 of your preferred search engine, the keyword is added. We have some more information at http://moz.com/help/pro/on-page-reports that should help.