Every country is allowed to send up to three athletes per event to the Olympics, and they can pick how the team is selected (trials, selection committee, etc.). But in order for an athlete to be able to participate, he MUST have the "A" standard for that event, UNLESS he is the only athlete from his country to go for that event, in which case he can have the "B". (Since we great Americans always have multiple "A" qualified athletes, the "B" rule never applies for us, really.)
Thus, the way our trials work is that we send the top three finishers who either already have the "A" or get the "A" in the trials. If you win your event at trials, but you don't have the "A", you don't go.
Keep in mind that basically all of our runners at 400m and shorter have the "A" standard by now. We kick ass in those events. Most of our 800m to 1500m athletes have it, and a good number of 5000m and 10000m as well. Steeple is probably our weakest.