Before revision:  Quotes of liquidly traded vanilla options on cross FX rates, e.g. EUR/JPY, can be used to extract a great deal of information about the complex implied correlation structure between the corresponding main FX rates, e.g. USD/JPY and EUR/USD. Including all this information in a ?nancial model means being able to fit simultaneously all volatility smiles, a very demanding task.
After revision:  Quotes of liquidly traded vanilla options on cross FX rates, e.g. EUR/JPY, can be used to extract a great deal of information about the complex implied correlation structure between the corresponding main FX rates, e.g. USD/JPY and EUR/USD. Including all this information in a financial model means being able to fit simultaneously all volatility smiles, a very demanding task.