There are two groups of coolant manufactured for passenger vehicles.
They are simply called type A and type B.
To simplify the difference with the two coolants would be to say that type B coolants are only suitable where aluminum corrosion and hot spot boiling are not a concern. This would make type B unsuitable for use in most post 1980 motor vehicles. Type A coolants give the best protection from hot spot boiling erosion and the inhibitors used are designed to protect a system containing a large amount of aluminum.
The component that controls the hot spot boiling erosion is glycol.So the main reason that type A and type B coolants have different capabilities to control hot spot boiling erosion is the % of glycol used. Almost all type B coolants have very little % of glycol when mixed in their recommended dosages. Types A coolants when mixed will have between 30% to 70% of glycol by volume. This % of glycol is required in the cooling system before any hot spot boiling protection is given.
Hot spot boiling occurs in isolated areas in an engine where the casting is thin and the temperature transfer high, such as the exhaust port waterways. Glycol's effect on these problem areas is to decrease the erosive action by reducing the size of the oxygen bubbles formed at these hot spots.
Glycol in type A coolants also acts as an anti-freeze. So glycol increases the boiling point and also lowers the freezing point of the coolant.