In fact, this method of expressing the thickness of paper is related to the knowledge of density: because the thickness of paper is uniform, we don't need to measure its mass per unit volume, only need to know its mass per unit area, which is the so-called "flatness" of matter.
According to the national standard, the paper format specifications are indicated by an, A 1, A2, B 1, b, etc. Among them, the paper size of A 1 is 84 1mmx594mm, and the paper size of A is 594 MMX 420 mm. Similarly, for linear substances with uniform thickness, we often only consider the mass per unit length, which is also called the "linear density" of substances.