For example, I need only the peak of a mountain, not the whole mountain, so i drag it onto the land and place so that only the peak is visible. Will the rest of the mountain use memory and processor?

You mean one of the MountainPeak01/02 pieces? It's one piece meaning either all or none of it is rendered. But it's a single piece so it barely takes up any memory... Unless you mean you dragged like everything off of a vanilla mountain. (Which is a really bad idea)

http://www.gamesas.com/user/699844-alexander-j-velicky/, in Skyrim there are many objects parts of which are hidden under land, so these hidden parts are rendered anyway? If so, i have no more question.

They are still rendered.

Ok, thx

Like I said, either the entire object is redered or none of it is. I don't know of a single game that would cull by only hiding part of an object.

Addition info: Even items that are 100% under ground are rendered. The ground in no way acts as a shield or window to direct culling. You can precompute occlusion, which makes it do this, meaning the game wont load LOD behind a giant mountain (As it is then a waste of space and power), but it never culls objects without you directly telling it to with occlusion planes, multibounds, and roombounds.

Oh this is really cool. I just saw how occlusion works with this game. It's easy to see if you're on the PC - go to any of the towns or settlements, in this example, I was outside in Falkreath. I disabled the Jarl's Longhouse by clicking on it with the console up, then typing "disable".

As I moved around invisible house, I can see statics behind the house (in relation to my view) appearing and disappearing. Cool stuff!

I prefer game engines with the BSP-style of occlusion where you had to run your map through a "compiler" of sorts to precalculate visibility though I understand that that only works for indoor style maps... not sure how that would work with wide open world games...