Hi!
I want to plot a surface parallel to the surface of a solid object.
I wonder if there are any specific function for that?
Are we talking Offset here? Or something else?
well, in my case offset would work also, if it can recognize the solid objects surface (not sure if I have explained properly) 
What do you mean by plot? A solid object, if not a sphere, is usually a polysurface, meaning that it’s composed of more than one subsurface. Do you mean to “offset” all subsurfaces of your solid?
A sketch or diagram could be helpful to better bring your ideas across.
Also what language are you interested in? Python? C#?
this is a extrusion of a surface (what i call a solid body) and i do not have that original surface (lets assume like that
)i want ro draw (create, plot) a surface parallel to the side of this body.
// Mazdak
Well a number of possibilities come to mind. It’s also not 100% clear if you want to do this via scripting or not, there are a number of ways to do this without a script.
First, if it is a boxlike structure as you show, you could simply use ExtractSurface with the Copy=Yes option and make a copy of that surface, then pull/move it somewhere else - it will remain on a parallel plane to the original. Otherwise you could also subobject select the surface (Ctrl+Shift+Mouse pick) and then use OffsetSrf to offset the surface some given distance (it will make a copy).
The same procedures can be done via scripting or Grasshopper, the principal challenge is getting the surface extracted/copied from the original body and in the case of offset, determining the offset direction.