senior member vs. registered user

So I am trying to figure out what all these designations mean -- new here.

I found the "upgrade your membership" page which explains some of it and the color coding, etc.

I've seen a lot of people that don't have any of those designations though and some of them say "senior member" and some just say "registered user". I take it that "registered user" means newbies like myself, or those that don't post often. But what is a "senior member" -- some of them are not color coded green/blue/yellow, etc?

Thanks for your help, just confused over here