The Stackup Viewer is an interface which functions separately from Sonnet. You do not need to have a project open to use the Stackup Viewer.
Like the Stackup Manager, it displays the technology layers, however it is not project dependent. The Stackup Viewer is used to view the dielectric and metal stack-up of a Sonnet project file or a separate Sonnet Technology File. By using the mouse, hovering over the dielectrics and technology layers, can give you insight into the properties of the stackup being viewed.