It is used for a variety of special situations. Many posters use B-series paper or is a relatively common choice for books. The B series is also used for envelopes and passports. It is widely used in the printing industry to describe both paper sizes and printing press sizes, including digital presses.
Just like the A series it is measured in mm and inches and the image below shows a visual example of how the sizes fit in with one another.
Why is it used?
This system gives a useful property for enlarging and reducing documents.