to be able to process as many gml application schemas as possible, the learn option is required because any new schema many and will define new object classes. These unknown classes have to be mapped to the viewers processing schema!
the viewer tries to find a model with the same default namespace as your gml data file. The use of this option will reduce the risk of mapping problems!
To achieve a correct mapping, there have to be previously known elements with the proper class within a property. Otherwise it may happen that the automised mapping process may attach a wrong java class!
If mapping problems occur, first create a new aristoteles schema file by save your learned model for a file you know the schema could be learned correctly and use this as new learning base during loading process.