When Google released "How Search Works" to the public for the first time, they also released its Search Quality Rating Guidelines. The Search Quality Rating Guideline is what's used to grade Google's Search results. Quality Raters don’t actually work for Google, but rather work for companies such as Butler Hill, Leapforce, and Lionsbridge. Lionsbridge claims they have over 4,500 people rating search results.
Quality Raters' work has only been seen when Google’s guideline document leaked a few years ago.
The document that Google released this week is considered an updated version of the old document that was released a few years. Only Webmasters and SEO's had access to the 161 page document. The newly released version was cut down to 43-pages.