It is important for molded rubber products to have certain standards. O-rings are no exceptions and there is something called an Aerospace Size Standard for O-Rings AS 568. It is a US standard for O-rings that came into being by the Society of Automative Engineers (SAE). As per this system, a dash “-“ is used to refer to O-ring sizes that are standardized.
The main benefit of having standardized system is that the engineers can make use of the standard O-ring sizes to build prototypes and test their designs. They would know the size of standard O-rings and they would not have to measure it every time. This brings a sense of uniformity. The Aerospace Standard mentions the cross-sections, inside diameters tolerances and size identification codes for O-rings that are put to use in sealing applications. Talking about the uses of custom molded rubber products like O-rings, they are used in the manufacturing world extensively. The biggest use of o-rings is that as an oil seal.
You must note that you can get standard O-ring sizes in different materials. The most common materials used to make o-rings are Nitrile, Neoprene, Butyl, Silicone, Fluorosilicone, Fluorocarbon, and EPDM.
AS 568 is Aerospace Size Standard for O-Rings to help engineers build prototypes and test designs. O-rings are used widely in the manufacturing world.