'Reputable Websites' for SSL inspection

What defines a reputable website? 

With the new Reputable Websites option in 5.4.1 I'd really like to understand what actually defines a reputable website. I understand from the documentation that it is a "secure white list database" of "reputable domain names that cain be excluded from ssl deep inspection".

When working with something that can potentially poke a very large hole in my security fabric I'd really like a better understanding of exactly how this works. For any Fortinet staff, if this is something proprietary a general answer would be great..

Although I'm looking for a general answer I'm mostly concerned with how sites that include user content are handled.