It's only at trials if a nation's trials double's as a national championship. If you look up U.S nationals you can find the B finals. And the Arena Pro Series has C and D finals. For example in 2009 there was B finals at the trials because they were our national champs, but this year they weren't because were having a separate national championship. Not all nation's have B finals as well, the US does and some others but not all. You see b finals at a level of competition below the Olympics and world champs basically, with trials constructed as their own mini Olympics for some nationals.
Fun fact: In 2009 international athlete's could compete at trials, but were relegated to B finals because the A final decided who went to the world champs. Since we had the polyurethane suits and crazy fast times that year, and because American sprinters were slow then, the B finals were faster in some cases.