Among law departments with lawyers stationed around the world, one goal might be to pay each lawyer an equivalent amount of salary, as adjusted for the cost of living in each location. If a certain amount of Euros paid to a lawyer in Rome buys a certain bundle of goods and services, that lawyer’s peer in Tokyo (all other things being equal) should be paid enough yen to cover the cost there of a comparable bundle. Hence the term, purchase power parity (PPP).
Relatively easy to say, much harder to figure out in the real world. Even so, law departments should strive for fair and equal pay around the world.