code: def subcategories_for_layer(self):
          
          purpose = self.parent.step_kw_purpose.selected_purpose()
          layer_geometry_key = self.parent.get_layer_geometry_key()
          if purpose == layer_purpose_hazard:
              return hazards_for_layer(layer_geometry_key)
          elif purpose == layer_purpose_exposure:
              return exposures_for_layer(layer_geometry_key)
docs: Return a list of valid subcategories for a layer.
             Subcategory is hazard type or exposure type.
  
          :returns: A list where each value represents a valid subcategory.
          :rtype: list