They are very complex. This thread below SOLELY looks at United fare classes ... A-Z and it is 3 pages long already.
All major airlines have a complex list just as long. You have not told us WHICH airline you are looking at.
<A HREF="http://www.flyertalk.com/forum/Forum50/HTML/014576.html" TARGET=_blank> Click for a detailed compilation - listing and outlining all known UA revenue/award/upgrade booking codes "A - Z" (and details and validity of many UA Upgrade Certificates) and how to access and view them on
www.ual.com </A>
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Glen ~ sipping bubbly from a UA 747-400 exit row 15 near you SOON!