Arnold 5 introduced a new closure parameter type, which shaders can output instead of final colors. There are BSDF, BSSRDF, emission, matte, transparency and volume closures. SItoA exposes the closure typed shaders and parameters as a new type, showing in purple in the rendertree.
The main goal of adding a new type, instead of simply recycling the Softimage color type, is not letting the user connect closures to color port, and vice-versa.
However, since the surface part of the Softimage materials is a color, we provide a special node, called "closure", that must be used to connect shaders such as the standard series into the surface slot.
The connector is automatically added between the Arnold shader and Surface when you create any of the standard shaders from the Arnold->Shaders top menu.
Please be sure to use closure only to connect the render tree into the Surface slot, and not in any other part of the tree.