DefaultTrust list are moderated by theymos. It is an account [1] . But users can make custom trust list by going trust list [2] . If he like to trust the users who are trusted by the user he adds to his trust list, he can change the trust depth to 1. If he want to trust users who were trusted by the users in trust depth 1, he can change the trust depth to 2 and so on, currently, it can be up-to 4 levels. I hope you understand.

[1] https://bitcointalk.org/index.php?action=profile;u=122551
[2] https://bitcointalk.org/index.php?action=trust

   ~~MZ~~