stands for Symbol Key . It is a unique, short text string assigned to every parametric component (fittings, valves, flanges, and equipment) within a Plant 3D spec.
AutoCAD Plant 3D allows users to customize how an SKey is visualized through the Symbol Manager. You can assign different 2D blocks (DWG blocks) to the same SKey depending on the project standards. autocad plant 3d iso symbol skey