All inputs and outputs of the HAL have a common set of properties and functions for reading or writing them. Some hardware channels have specific properties not common to other channels, e.g. a pulse width modulated output needs to be assigned its base frequency. Another example would be a digital to analog converter which has a calibration function which needs to be called upon request.
Feature functions are functions which are specific to a HAL object. They can be called by a user with

  void callOutputFeature(OutputInterface *obj, std::string featureName, ArgTypesOut... args);
  void callInputFeature(InputInterface *obj, std::string featureName, ArgTypesIn... args);

The parameter are as follows:

1. OutputInterface / InputInterface: the HAL object
2. featureName: name of the function as defined in the appropriate hardware wrapper library, see Hardware Libraries
3. args: arguments to this function