Rendering is the act of converting the scene information into a bitmap image by evaluating all the geometry and lighting information in a given file. Generally, you'd expect the resulting rendered image to be a fully ray traced and shaded image with full shadows, reflection and the like, much like a photograph depicts an entire scene when taking a picture with a camera. In Modo these images are the result of what are called "Render Outputs", the visible representations of the full scene evaluations. Each Render Output is a separate layer in the Shader Tree and produces its own image layer as part of the render calculations. Every time a Render command is invoked, all of the render output layers are simultaneously calculated and represented in the Render Display viewport. The different layers can be viewed by choosing one of the specific 'Output' options within the display window.
Occasionally, only a subset of the full evaluation is required, such as the diffuse color layers without any shadows. Modo offers many different Render Output types useful in a variety of situations ranging from purely diagnostic, to specific compositing utility in an external application. A default scene automatically includes a Final Color and Alpha output which should be fine for most situations. Additional layers can be added from within the Shader Tree itself, by clicking the Add Layer option of the full viewport window and selecting Special > Render Output from the pop-up menu. This adds an additional layer to the top of the tree. Users may then right-click in the Effect column of the Render Output for a pop-up context menu that allows one to select from the alternate output types. See the Render Outputs topic for for information regarding the editable attributes of the individual outputs. For more information on the render output types, see Render Output Items.