They are randomly generated based on the number of posts you have, and I cannot think off the top of my head if there is anywhere that lists the levels.
You can change them here:
https://www.mobileread.com/forums/pro...do=editprofile
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Custom User Title This is the title that appears beneath your name on your posts. Check the 'Reset' box if you wish to use the default title for your usergroup and post count.
Your current user title is
Irrational Optimist
Custom User Title
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