What is fungibility

In economics, fungibility is the property of a good or a commodity whose individual units are essentially interchangeable. 

For example, since one kilogram of pure gold is equivalent to any other kilogram of pure gold, whether in the form of coins, ingots, or in other states, gold is fungible. Other fungible commodities include sweet crude oil, company shares, bonds, other precious metals, and currencies.

Fungibility refers only to the equivalence of each unit of a commodity with other units of the same commodity and not to the exchange of one commodity for another, which is barter