defining XML

Hi,
i am creating an application on the client end(in HTML/Javascript) from which the user selects a map and data corresponding to that map. for eg, he selects the map of USA and then selects a data file containing the area and population of various states in USA.
now an XML file is to defined. this XML file should indicate the user selection ie if the user decides to calculate the density of the various states of USA and therefore selects the data files containing the area and population of some of the states, the XML file should reflect this selection.
this XML file is then transmitted to a server. on the server, a servlet/JSP interprets this XML and performs the calculation like finding the density of population(by dividing population with area).
now my question is: how can an XML file be defined in such a way that it indicates the user selection and that can be transmitted to the server?
Thanks in advance.