Thank You gr5, for Your answer.
I am familiar with outer and inner walls, but think is about skin, Cura 2.7 is the first time I see this parametr (normally i use repetier), but becouse of lack of the parameters (in Slicer nad Cura engine), I decided to move into Cura for slicing models.
My idea for skin is that Cura call outer wall - skin? But I am now sure about that.
Radek