Views contain both real and aggregate resources (nodes and links), which are graphically represented as icons (or shapes) and lines. The icons or shapes for aggregate nodes have plus signs (+) in the lower right corner. The lines for aggregate links have plus signs (+) in the center of the link.
An aggregate resource represents a collection of real or aggregate resources, or both. The status of an aggregate is a reflection of the status of its underlying real resources.
You can set the thresholds at which an aggregate resource changes status by selecting it in a view, clicking the right mouse button to obtain the context menus, then opening the Resource Properties notebook to the Aggregation page.