The caliber is a unit of length based on the inch (imperial units) and uses the symbol cl.
It represents the internal diameter of a gun barrel and therefore more often than not, the size of the ammunition or round that gun requires to fire.
It is not always represented or converted exactly in inches so a table or "well-known" conversions are often used to depict measurements / ammo types instead.
The centimetre is a unit of length and is a multiple of the metre; an SI derived unit. It has the symbol cm.
A centimetre is equal to 0.01m. It is also ≈ 0.3937in.
Centimetres are used in maps to scale up conversions to real world measurements.