It's like this...For those airports that have the CTX machines (x-ray for checked baggage) if there is any part of the bag that the machine and/or the screener is unconfortable with, the bag is subjected to physical search. Whatever is determined uncomfortable is at the discretion of the screener operating the CTX at that time. Some screeners are more "paranoid" than others. For lower volume airports (i.e. Category III and IV airports) all baggage is subject to hand searching, becuase the CTX machine is not present at thoise airports. The baggage is still electronically screened for explosives, but just by hand. At this time, TSA cannot justify placing a 1 mil+ dollar machine at an airport that only screens up to 500 checked bags a day when one or two $50,000 machines can do the job instead.