The projection is set by the RiProjection procedure, and defaults to an orthographic projection. A perspective projection is the kind we are normally used to, with a vanishing point at the horizon, and where objects shrink as they get farther away. Objects are projected onto the screen plane along lines that intersect at the camera and get farther apart with distance. An orthographic projection is a much simpler projection. They don't go through the camera at all, unless you like to think of the camera as being an infinite distance behind the screen plane.
Objects appear the same size on the screen independent of their distance from the camera.
It should be clear that we can draw such projection lines to any object which is in front of the camera, and thus all objects project to the screen somewhere. However, we don't want everything to be in the image, so we only display those objects which project into a certain rectangle of interest on the screen, the so-called screen window. You can think of the screen window as being the rectangle which is the intersection of the perspective viewing pyramid or orthographic viewing box with the screen plane. The screen window is set by the RiScreenWindow procedure. Generally, the screen window is centered at the origin.