Blackmail's reply is not entirely correct, or not in the UK anyway. An aircraft can be considered to be at a level when three successive radar sweeps confirm that it is at the level +/- 200 ft and so 300ft or more is technically a level bust, this is also true in RVSM airspace. As Alan M says it is more often than not a high rate of climb which makes the TCAS blow as it assumes you will keep climbing/descending at that rate. This is a big problem in TMA airspace when we are trying to achieve a lot of climb throughs in a small piece of airspace and therefore often ask for high rates of climb. Having said that, I have seen RA's between two straight and level ac very nicely 1000ft apart. I don't think you will get caned for 2 or 3 hundred feet as long as it is rectified swiftly and the one on top of you hasn't overcooked the descent by a similar margin !!