Just magically after posting this, a thread with the same question and the answer showed up. ] :
viewtopic.php?f=3&t=13132&hilit=perlin+noise
[I recently found a link somewhere in the forum to a file which explained the different variables when registering a biome.
Unfortunately I can't find it anymore, not in the forum, nor by using a search engine.
I found the reference/tutorial very helpful and I would greatly appreciate it if someone could come up with the link to it.]