Tutorial 3dsMax -- CrossSection Modifier
The CrossSection modifier creates a "skin" across multiple splines. It works by connecting the vertices of 3D splines to form a skin. The resulting object is another spline object that can be used with the Surface modifier to create a patch surface. These two modifiers, when used together, are sometimes referred to collectively as “Surface Tools.”
Sometimes completely unknown, the modifiers are very useful during the normal workflow of 3dsMax. These are old videos created with 3dsMax2010 but the modifiers are equal to the latest versions