Even though several options are available, most of them are only there for compatibility reasons. For stereoscopic double images you should always (with the exception of special circumstances) set Placement to Off Axis. Each example in the image above uses a double camera only. In the Render Settings’s stereoscopic settings, images can be set to render in channels, i.e., a scene can be rendered from any number of camera perspectives. Their arrangement will also be as shown above.