Wow @GregValiant THANKS for that thorough explanation.
You're saying that the layers printed on the bottom and top of the model, excluding the inner and outer walls are the skin (am I correct ?). I'm still confused though, under the shell category there is no way to set number or thickness of skin (just top surface skin layers), but I can change Top\Bottom thickness. I'm concluding that there are Top\Bottom layers (adjustable thickness ) and skin layers (non adjustable thickness), my question is what is the difference or relationship between Top\Bottom and skin ?