You can't assign values to everything. The first letter generally gives you the booking code (QUP fares are a notable exception). An N in the fare basis code generally indicates a non-refundable fare. X often means a weekend fare (or maybe the other way around - I can never remember this one). Numbers in the code generally indicate the advance purchase restriction.
Originally Posted by hakgwai
PS. noticed that the us airway website has flights cheaper than UA. even its a codeshare
Sometimes yes, sometimes no.