the OP was under the impression that *only* conventional, 3-letter codes (e.g., ORD, IAD, LGA, etc.) were used s/he would be unfortunately inexperienced of the brutal realities.
One sees "CHI" as in "Chicago" but for either ORD or Midway , "WAS" for Reagan or Dulles, etc. and NYC for EWR, LGA, JFK and from what I hear, as far afield as BWI.
Egad!
Oh, the horror, since some serious confusion can result.
Worse, my credit card statement has an odd mix ---LAX CHI YTO--of conventional and non- codes. All the more curiously because NO-one conected with any tix I charge uses those #$%^ codes, not aircarrier reservations staff, not airport staff, and certainly not well-seasoned FT-ers.
Sooooo, WHERE do they come from?
(But I don't really
want to know.
)