@Udo:
ok, I get it.. and thanks 
Let me explain one other concept around this: ok, I'll read the whole core and ok, I see what are all parts setted (i.e.). But for every settings, I have to check on the datasheet. To understand very well the PWM (all modes of operation, all exceptions and how are they handled) it takes a day or even more (because when you try to write code not always it works, and for that reason I wrote a little doc file, a draft, where I noted all combinations and their exceptions. In this way, further codes are written in an easily way). Now think if you should do this for everything.
What the Arduino does, it's the same work to categorize all functions in object of C++ (that I done for PWM in a text file) and reuse them in an affidable and clearly way. So, even if you read my document, if you don't have a datasheet, you will not understand.
BUT IF you would know simply "oh, cool, my led fades: what's the default frequency and what its default phase and interrupt modes?" if the answer will be "you should read the configurations adopted and, after you understand the code, then you should see in tables what are those registers setted", it's different from "you have to know what you are using because it's not written by you, so: the frequency by default it's 490Hz, phase-corrected mode, interrupt on top disabled. BUT if you would like to change it, let's start to study".
Now I shown to you an example of what I looking for, and I try to explain why understand the code it's only the 50% of total work, specially for people like me who don't are experts in AVR MCUs.
I hope to make the main idea.
Maybe in books written for Arduino it's probably written something about that. And the other thought was on WHAT it's loaded when I simply blink a LED (it's more than twice, and more than twice of twice when I set the serial interface). I can't understand it, maybe it's written somewhere? Because if I can't find those infos,for my purposes all the Arduino.cc became useless and I should start to study the case in a professional way, which is don't the case. And all for the only reason that they aren't able to tell me what are things that they had written. It's hard to believe to me.
PS: can you tell me a books title or documentation which talk about this?
thexeno