The primary purpose of the up and right vectors is to tell POV-Ray the relative height and width of the view screen. Also the angle calculation depends on the right vector so right should precede it.
Most camera types treat the up and right vectors the same as the perspective type.
However several make special use of them. In type 3 the up vector determines how many units high the image is. All viewing rays are perpendicular to the y-axis. For type 2 and 4, the cylinder lies along the right vector. Viewing rays for type 4 are perpendicular to the right vector.