It doesn’t really matter if you know your Linear Algebra. The difference is only a negation on the W vector when calculating the directions.
This is all relative in some arbitrary units. This has nothing to do with absolute “screen size”. Screen means client window size in pixels (or render resolution in cells) in that case irrespective of their size in the physical world.
None of the OptiX examples knows what monitor size you’re running on or what the size of a pixel is on it. (Just the aspect ratio calculations assume pixels are square.)
Again, the UVW coordinate system is not an ortho-normal basis, means none of the three vectors is necessarily normalized, nor do they need to be orthogonal. (If it is you have a camera with 90 degree field of view on both axes. Perfect for rendering cubemap faces. I digress…)
Means the camera plane can become a parallelogram and the projection can be sheared if you set the UVW vectors accordingly.
(Sheared view frusta can be beneficial for stereo setups to reduce ghosting due to foreshortening between left and right image.)
Picture this: The UV coordinates are spanning a window into the virtual world. The ratio of U/V is normally the same as the aspect ratio of the render resolution width/height.
The vector W points from the camera position, which locks that coordinate system into space, to the center of this virtual window to the world.
Since W is not normalized, you can change the field of view by changing the length of W while keeping UV fixed. Or you can change the field of view by keeping W fixed and changing UV.
(Means you can make the UV vectors half your render resolution size and adjust the field of view by the length of the W vector if you want.)