Hmmm...in my macro or money and banking texts, targets were interest rates or money supply. One could add exchange rates, I suppose, as a target, but typically this would be thought of as an after effect. China, with strict captial controls, may indeed target the exchange rate, but this is so far from an open economy with a central bank that it cannot be compared to Europe, Japan, and the U.S.. Explicit examples are also currency boards, though tyically these simply fix the exchange rate, rather than targetting the exchange rate, allowing their currency to rise and fall with the dollar. This is similar to the highly successful (for a while) Bretton Woods system.

Bottom line is there is one degree of freedom, and that is the money supply. Whether this is adjusted to keep interest rates at a certain level, or simply constrained to grow at a certain rate as during the Volker era, completely ignoring interest rates, is interesting. The value of the currency will reflect monetary policy, for sure, and is certainly not independent of money growth or interest rates.

For instance, if the U.S. seeks to counter Japan's attempt at devaluation, it could try and grow its money supply (not just the base) at a rapid rate, keeping the dollar from appreciating by fostering domestic inflation. This of course, would exacerbate inequality, since the rents from this inflation would go, as usual, to capita.