Why is permeability number important?

Hint: The permeability number is calculated by the rate of flow of air under standard pressure. it has no unit.
Permeability may be measured by the commercial equipment and is generally denoted as a permeability number.
 We have an idea about magnetic permeability during the study of magnetism.

Complete step by step answer:
The permeability number is the numbered value related to the molding of substances indicating the relative amenities of the passage of gases through them. It is calculated by the rate of flow of air under standard pressure. It is a unitless quantity. The commercial equipment can measure the permeability which is known as a permeability number.
The increase in permeability generally implies a more open structure in the rammed sand. If this increase continues, it will become penetration-type defects and rough castings. A decrease in permeability denotes tighter packing. It can lead to blows and pinholes.

Note:
In the part of Magnetism in physics, we can know about the magnetic permeability of a material. When the air is replaced by a substance, the ratio at which the magnetic field is changed in the substance is called the relative permeability of the substance.
For a solenoid of \[n\] numbers of the coil, if $I$ current passes through the solenoid, the magnetic field \[\left( B \right)\] occurs along the axis if the air is taken as the core of the solenoid.
$B = {\mu _0}nI$
${\mu _0} = $ the permeability of air.
Based upon the magnetic permeability, the materials are divided into three parts: Diamagnetic, Paramagnetic, and Ferromagnetic.