I don't know if UAL still does this but back in 97 when I flew them a lot in the summer I remember they had this thing called the B/C system for gates at O'hare. If you the a/c was either bound for, or arriving from Boston, ATL, LGA, EWR, PHL, DCA or MSP it was called a B flight and assigned a B gate at the B concourse. All other destinations were C flights and went from the C concourse. The thinking was the high paying business travelers were usually on the B flights, so why not allocate the closer in gates for those types of pax. In the evening and early morning times flights bound for SFO and LAX also became B flights and launched from the B concourse. Of course you would see flights going to or arriving from OMA, DEN, IAH, DFW, STL to name a few on the B concourse, but 90% of the time that exact a/c was either continuing on to one of the cities named above or coming in from one of those cities and going to a "C" city. Like I said before, I don't know if UAL still sticks to this schedule or not, and of course when the bad t-storms would hit then and they needed every gate they could get, it didn't matter where flights were coming or going they went to the next availble gate whether it was on B, C or E!