Pablo, you got that the wrong way round:
TRANSITION ALTITUDE is used during the climb when you have your altimeter set to QNH (ie local mean sea level pressure- so you get your altitude reading in relation to local terrain right)- at Transition Altitude you switch to STANDARD SETTING 1013mbs/29.92 inches so that everyone is flying to the same setting.
TRANSITION LEVEL is used during the descent when you have your altimeter set to 1013mbs/29.92 inches, and you want to get nearer the ground so it makes more sense to set local QNH so you can get an idea of your proximity to local terrain. Transition is at 18,000'/FL180 in the US, and usually about 6000'/FL70 in the UK- you just look on the airport approach chart to see what it is. The difference doesn't matter- you just do it.
That some people persist in going to QFE (local airport pressure- so altimeter reads zero on the ground) completely mystifies me. Your chart shows terrain up to xxxx feet- having QNH set will instantly tell you whether you are likely to bump into it-in short, QFE is useless, but it has its fans for some strange reason! I'm far more interested in knowing my height over terrain than my height above a runway when I'm threading through variable terrain!