I'm pretty sure it's based off of the configuration in the green gear in the top right of the iframe. You can inspect the ways it's calculated from karma, comments, etc etc
If you look at the config it's based on karma, comments, submission rate, comment rate (optional), and account age (that is, if you trust it actually uses the config how it says)
If you look at the config it's based on karma, comments, submission rate, comment rate (optional), and account age (that is, if you trust it actually uses the config how it says)