The type mentioned (Non-Ruled/Plain) is blank paper, typically without pre-printed grids or ruler lines.
It is commonly used for publishing professional manga in Japan. 22363mp4
Standard professional B4 manga paper measures 250mm x 353mm (roughly 9.8" x 13.9"). 22363mp4
Here are the details regarding the often used for detailed illustration: 22363mp4
It is often paired with digital tools like CLIP STUDIO PAINT for drawing and finishing, according to community discussions on CLIP STUDIO ASK .