On the Regions panel of the Viewport Configuration dialog, you specify default selection rectangle sizes for the Blowup Region and Sub Region rendering options.
The rectangular selection region appears in the viewport when you choose either the Blowup or Region option from the Area To Render list. You can change the size of a region by dragging its handles, and its position by dragging anywhere within its borders.
To use the virtual viewport:
The viewport is converted to a virtual viewport and displays the area of viewport represented by the white rectangle. All viewport navigation methods work the same, except that you're seeing only the zoomed portion of the viewport.
Sets the position and size of the blowup region. See Area to Render.
Sets the position and size of the region to render. See Area to Render.
OpenGL viewports have Virtual Viewport options that let you zoom in on a sub-region of the current viewport, creating a “virtual viewport” where you can perform any standard navigation, but in a zoomed-in area. For the Direct3D driver, these controls are disabled. When Nitrous is active, these controls don't appear at all.
You can use the virtual viewport on any type of viewport, but it's primarily designed for zooming in on camera views. This lets you perform close-up work, such as tracing, without distorting the relationship between the geometry and a bitmap background. (With the standard Nitrous viewports, 2D Pan Zoom Mode provides a similar function for camera, perspective, and spotlight viewports.)
Because you're actually zooming the viewport image itself, the viewport label might be hidden from display, but you can still right-click in the upper-left area of the viewport to display the menu. This takes advantage of zooming features in the OpenGL driver so that 3ds Max does not compute the display change internally.