Shape Key - A data structure that represents a shape key of the object.
Object Socket - It can intake or output the object(s). The object input socket has two eye droppers, the first eye dropper icon will allow to select object from 3D viewport or outliner. The second eye dropper icon (solid) will set the input object to the active object in the 3D viewport, and hold CTRL to open a rename object dialog. This socket has node link conversion for object to vector, matrix, mesh and
Shape Key Socket - It can intake or output the shape key(s).