All of the layers you add to an NPMap web map are handled through a layer handler. Do you need to decide which layer handler to use in your web map? This is an important decision that requires careful planning in order to ensure your web map is visually pleasing, usable, and performs well.
This video quickly goes over five of the eight layer handlers NPMap offers. Support for GeoJSON, JSON, and XML layer handlers are coming in the next release of NPMap. You may also want to take a look at the layer handler support documentation here: http://www.nps.gov/npmap/support/layer-handlers.html. It contains the latest information about NPMap's layer handlers.