The original meaning of the word "secondary element" is a two-dimensional plane and a concept of spatial dimension. Any point of the second dimension can be located by two coordinate axes (such as X axis and Y axis), and the area of the second dimension is usually expressed in 1 pixel.
In Japan, early animation and comic works were all composed of two-dimensional images, and these images were flat, so they were called "two-dimensional world" by animation lovers, as opposed to "three-dimensional world", that is, the real world.
The ACGN cultural circle uses the word "secondary element" to mean "overhead", "imagination", "fantasy" and "fiction", and then refers to "overhead world".