Hill assist is a mechanical system. It uses a linkage from the clutch pedal, that connects to a mechanism/cylinder plumbed in between the master cylinder and ABS modulator, only on 1 of the 2 circuits. (think front right and rear left?) Least thats where it used to be on my forester before i chopped it out and fabricated new brake line to bypass it.

im 99.9% sure its for the headlight levelling. It measures the rear suspension hieght and uses that to determine the required angle of the headlights. The idea is that when you load a car with cargo/passengers/fuel, the weight typically is added to the rear. Therefore it mostly affects the rear hieght. Whereas the constant weight stuff (engine/gearbox/driver/ 1/2 the car body) is mostly over the front axle and doesnt alter. So they use the rear height change to determine headlight angle.

Most subarus with HIDs have this feature, as it is needed for ADR approval. Some of the early HID Suabrus (like 04/05 JDM Forester with HID) have a manual adjustmemt switch.

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Doing it for all the wagons out there