A HAL always includes header file with function declarations for the HAL functionality in /hw/hal/include/hal. The first argument of all functions in the interface typically include the virtual device_id of the device you are controlling.
For example, in hal_gpio.h the device enumeration is the first argument to most methods and called pin.
void hal_gpio_write(int pin, int val);
The device_id (in this case called pin) is not a physical device (actual hardware pin), but a virtual pin which is defined by the implementation of the HAL (and documented in the implementation of the HAL).