Why does DQN can have custom function?

I am reading the link: ray/parametric_actions_model.py at master · ray-project/ray · GitHub. The class TorchParametricActionsModel(DQNTorchModel) is inherited from DQNTorchModel. It is bit confusion that since the custom function is supposed to be policy function. But DQN does not have explicit policy function. Anyone have more knowledge on this? Thanks in advance.