Window > 5-First Project > Lesson 5.20: Preview Rendering "Any time you want to quickly examine changes in lighting or materials, click the Render icon in upper right corner of the Document window. Your render preview will appear in the Render tab.

Preview renders will not be as detailed as final renders. For example, they don't include reflections or more advanced material settings. However, preview renders are used frequently to check lighting and material changes that you cannot see in the document window."

Makes sense to me; however I have no experience with any previous versions, so if this is the way it is for everybody then so be it. Documentation doesn't fit the intended functionality.