def get_spatial_unit_conversion_session_for_type(self, source_spatial_unit_record_type=None, target_spatial_unit_record_type=None):
Gets an ``OsidSession`` associated with the spatial unit conversion service and the given spatial unit record types.

        arg:    source_spatial_unit_record_type (osid.type.Type): the
                type of the source spatial unit record
        arg:    target_spatial_unit_record_type (osid.type.Type): the
                type of the target spatial unit record
        return: (osid.locale.SpatialUnitConversionSession) - a
                ``SpatialUnitConversionSession``
        raise:  NullArgument - ``source_spatial_unit_record_type`` or
                ``target_spatial_unit_record_type`` is ``null``
        raise:  OperationFailed - unable to complete request
        raise:  Unimplemented - ``supports_spatial_unit_conversion()``
                or ``supports_visible_federation()`` is ``false``
        raise:  Unsupported - ``supports_spatial_unit_record_types_for_c
                onversion(source_spatial_unit_ record_type,
                target_spatial_unit_record_type)`` is ``false``
        *compliance: optional -- This method must be implemented if
        ``supports_spatial_unit_conversion()`` and
        ``supports_visible_federation()`` are ``true``.*