Which are the modern and the traditional exchange rate theories? What distinguishes them? What is the relevance

Question:

Which are the modern and the traditional theories? What distinguishes them? What is the relevance of each? What is the relationship between them?
Exchange Rate
The value of one currency for the purpose of conversion to another. Exchange Rate means on any day, for purposes of determining the Dollar Equivalent of any currency other than Dollars, the rate at which such currency may be exchanged into Dollars...